Browser users are not (by default) Atlassian ICPs so IMO there's zero lock-in. I am going to most likely change my browser very shortly because I don't see Atlassian building out Arc. TBC raised $50M, they and their investors got a good return in a short amount of time. This chapter is now closed.
Focusing on a single db was not the original intent as migrations was a big reason why the ORM product category became (and still remains) popular.
As for performance, you are right. We do take a hit when trying to support as many DBs as possible. However, performance is not an issue. Take a look at https://benchmarks.prisma.io. Regardless, we're always open to being given examples whereby we can dig into performance related issues. So if you have any, please send them our way.