Dorico should mark the remaining 2 crotchets in the example as an explicit 2/4 automatically
Doing this would wreck Dorico as a film scoring tool unfortunately. It would make so much extra work for film composers that they would stay on the old version to avoid this, or move to something else.
It isn’t correct notation, but this gets corrected through the writing process.
Just to elaborate, a common process for film scoring is that composers will start by coming up with the tempo or tempi for the scene even before they’ve written a note, find the right tempo to match the pacing of the scene and get as many hitpoints as possible, possibly entering dummy notes at these hitpoints. They may not start scoring the scene from the beginning, but instead from near the end and fill in the earlier part later (same as normal composition). This means that the end of the music may be written already and in perfect sync, but the first part is not yet written and is just a bunch of empty bars - albeit with the tempo already decided and with enough beats to get to the end. If these are 4/4 and when you start writing you find that the music you want to write for this is not 4/4, obviously you have to change this, and here’s where it can theoretically start to become a problem (although not a problem in DAWs and not a problem with Dorico under the current behavior).
The issue with the initial suggestion of having Dorico just add time or remove time without asking is that your previously composed music later in the cue is in perfect sync, and this sync would be thrown off by the addition or removal of time, which would not be acceptable. Instead, the current workflow/behavior is that, after writing the end or whatever part you wanted, you might then work from the beginning, changing time signatures as needed, then when you reached the spot where you had written to the end, you would see how many beats were left (as you would probably end up with a fractional bar there) and add the correct time signature so that everything would be musically correct and complete. This can all be done without impacting the synchronization of the later music that was already written whatsoever. If the software were to automatically add explicit time signatures to incomplete measures, you may have to go through a process at the end to delete any extra time signatures it auto created because you may not want it to be like that, and deleting the time signatures may further throw things off. It could make a big mess of extra time signatures created through this automatic process that you suggest, if this was done repeatedly - for instance if you changed a time signature, and it created a new fractional measure with its own explicit time signature later in the piece, then if you changed another time signature in the next bar, it could make a new fractional measure with its own explicit time signatures later in the piece. If you change the time signature 10 times, you can easily end up with 10 shorter measures with random time signatures that you don’t want that were auto created by Dorico when earlier time signatures were changed.
The only thing Dorico does differently than a DAW is that in a DAW you won’t usually end up with these “fractional” measures like in Dorico. The reason is that, to keep the notes at the same timestamp, the DAW will often offset the actual notes in the bar - in Michel’s example, it would be as if he changed 3/4 to 2/4, and all of the music in all future bars shifted back by a quarter note to compensate so that they landed at the same timestamp. These rhythmic shifts would be harmless in a DAW and easily correctable, but in notation, this could easily screw up custom engraving that was done, and applying a corrective shift back after the fact may not restore things back to how they previously looked. This is similar to how you can wreck certain engraving details by shifting things in insert mode and, even after shifting them back, they may not look exactly the same as they did before they were shifted in the first place. So instead of adjusting the actual notes forward or backward in time to keep the timings a match, Dorico handles this slightly differently from a DAW by having these fractional measures. I can understand why it is done this way, as a bit of a compromise as the normal DAW behavior would be too destructive of any custom engraving that was done.
There has to be a way of improving things that doesn’t mess up the current film scoring workflow (which actually works pretty well).