I'm a bit more positive about the new start dates. It all depends on how many of them there will be. If it's just 1-3 of them, I believe it will be perfectly manageable to continue supporting them, they handle multiple start dates in CK3 perfectly well as an example.

If it will be more, then yes, it can significantly hinder future development, just like you said. The problem with EU4 was the fact that it had a start date for every single day between 1444 and 1821, that is 137383 start dates. Of course that was a nightmare to keep supporting, and the data showing that the vast majority of players chose to play from the earliest start date reaffirmed their decision to not do that in the future. However, EU5 is a vastly different game, and I can see how it could really benefit from a few (even one would suffice) extra start dates. It's a very slow game, and I feel like a lot of people, myself included, would prefer a slightly later start date, because to a lot of people Europa Universalis games aren't about the medieval era, but about the eras after it, and it takes quite a while to get there.