A decade ago is when they started the transition. It’s been painful.
If you’re a new shop that is making decisions without looking into how the company that pretty much runs the platform you’re basing your future on has acted in the past decade (we’ll ignore how they’ve acted beyond that because then it’s a no brainer) then you’re doing yourself a disservice.
I see that you’ve narrowed the goal posts to just technical concerns, which is fair, but isn’t sufficient to make a decision about what technology to choose.
Especially in a field where you have a similar alternative in Java where the sponsoring company doesn’t have half as much control, as well as several fully open source alternatives.
It sounds like you’re projecting the problems of an existing .NET shop onto the shape of a startup without all that baggage. I can assure you, having worked with many customers running new business on newer .NET, it hasn’t been a legit technical concern since about .NET Core 3.