Well, not everything. I’m decidedly missing some key stuff that many people have – Spectrasonics Omnisphere is one huge omission in terms of ultra-popular instruments. And there are some newer ones that many have, as well. However, my music is mostly more on the organic side (acoustic drums, bass guitar or acoustic bass, acoustic and electric guitars, piano, and sometimes organ as the core), adding flavor – synths, strings, brass, ethnic instruments – only if needed for a specific song, or if I’m intentionally working on something out of my comfort zone, for example to target a specific opportunity.
I originally got into AAS because their Tassman was included with a Cakewalk DAW (don’t recall if it was Pro Audio 9 or SONAR). The rest of the stuff (I no longer have Tassman as the version I had hasn’t worked for ages on my systems) I have of theirs I likely picked up from loyalty deals and because, at the time, they sounded like they might be of interest for some basic need or specific project. Chromaphone has really been the only one that’s stuck with me, and I still do at least try it out when fishing for potential sounds for a synth application where I don’t already have a good idea of what I need. I find it interesting for sounds that aren’t necessarily recognizable but have a textural element, or possibly as one ingredient for a combined patch.
That’s one way to avoid the temptation. 
I’ve recently unsubscribed from a few email lists, but I still keep up most of mine, even the ones that get annoying due to sending multiple emails a day (UAD and IK are especially guilty of this; at least Waves tends to limit their emails to once a day) because, ultimately, I do want to find out about their new products, and they don’t seem to have options to limit the types and numbers of emails they send you – you know like, don’t keep mailing me sale ads for products I already own!
The new capability and better-than-current capability have mostly been my criteria on additions. On the latter front, I still keep the older plugins and instruments around, both for compatibility with older projects where I’ve used them (at least as a starting point when doing a remix – often my main reason for a remix is to significantly upgrade the sound quality to target a specific opportunity with a song whose recording needs to be updated, and I’ll end up replacing specific instruments and most of the audio plugins) and to have options on flavors.
I don’t see having multiple bass plugins as redundant. If you just consider bass guitars, there are many flavor possibilities. While I mostly use MODO BASS 2 for that right now, sometimes a Kontakt bass guitar may be a better fit for a specific project or genre. And, of course, maybe a different type of bass is needed, like an acoustic bass (MODO does now do those, but maybe Trilian’s might be better in some cases?), and, then there are synth basses, where there can be lots of different characters for different flavors, and then there can be layering of different basses for different elements of a composite sound.
Of course, this kind of consideration comes even more into play with guitars (various flavors of electrics and acoustics, strumming versus picking versus leads, what kind of inflections might be needed for a part and how much programming might be involved, etc.) And the reason I’ve mostly standardized on PianoVerse MAX for acoustic pianos these days is because that one plugin provides so many high-quality flavors, though there are still cases where I need something else, like NOIRE for a felt piano or some other unique instrument.
For me, this comes down to the types of music you make, the needs of that music, and what products fulfill those needs most all the time versus a fair portion of the time versus sometimes.
So, for example, in my case, typically making organic music, Superior Drummer (plus a whole bunch of SDX and EZX extensions) fills my needs for drum kits almost 100% of the time these days, so it’s a no-brainer to load that up when I need drums (which is probably 95% of the time). It sometimes can fill percussion needs, too, but nowhere near as consistently. On that front, I’m more likely to go with either In Session Audio’s Shimmer Shake Strike 2 or NI’s Session Percussionist. (The latter is pretty limited in terms of time signature and measure boundary considerations – it really wants even number measure boundaries, so, if you throw in an extra measure for emphasis at some point, it can be hard to deal with.) PianoVerse can fill my needs for acoustic pianos in most cases nowadays. But when it comes to guitars, I’m still using a bunch of different product lines – MusicLab, UJAM, NI Session Guitarist, and Acoustic Samples – because, not only am I using multiple types of guitars in most recordings, but there are also different needs for each part for both sound and type of playing. When I’m doing a brass section (sax, trumpet, trombone), I almost always use the SampleModeling instruments, which I play with a wind controller. But when it comes to synths, that can be all over the place, and I’m more likely to experiment, be it just within Analog Lab (which can bring up sounds from all the V-Collection instruments) and/or trying other developers’ products (e.g. Chromaphone, Padshop, Massive, etc.).
Yes, that is largely true. I am recording songs I’ve already written the vast percentage of the time, and I have a feel for what they’ll need when I record them, even if I’ve only played them piano/vocal to that point. It’s pretty rare for me to use a DAW as a compositional tool, and it’s also pretty rare for me to look to sounds for inspiration for writing a song. I have done a little of that, mainly for instrumental song opportunities, but it’s not really my thing – essentially, I’m a song person first, with the recordings subsidiary to that. That said, each project also does get into some experimentation, for example for choosing guitar sounds and processing, and, if is something that will need synth sounds, then there is a lot more experimentation involved.
