There will always be an update later!
You have to judge whether the additional functionality added in Dorico 6 is worth the modest update price we are asking for it.
If you bought Dorico 5 as a crossgrade from Finale, you paid only $149 for a product that would normally cost at least $299 (if you bought the regular Finale crossgrade) and for which many people paid as much as $579, so you got a great deal.
Do the features added in Dorico 6 provide $99 worth of value for the things you do with the software? If so, great. If not, eventually (though not for a good while) it will be possible to buy an update to Dorico 6 at a special price or a sale, and you can jump on then.
In the meantime, your copy of Dorico 5 works just as well today as it did yesterday, and will continue to do so.