The GCP Pub/Sub API has largely replicated all the features you'd want out of Kafka (including Consumer Groups). The primary consideration at this point is cost. There's an inflection point in size (at some very large message volume) where it makes sense to start running your own Kafka cluster and hire a dedicated person or two to manage it. Most companies will never get anywhere close.

Any project just starting out should use Pub/Sub. One thing I really like is that GCP provides emulators of Pub/Sub et al for local testing. That used to be a bit of an obstacle not too long ago.

In terms of lock-in, I don't see how that applies to an AMQ. The data moving through it should only be transiently persisted, up to a week or two at most in the usual case.

If you want to avoid cloud lock-in, have DB backups, use Postgres/MySQL/etc, containerize your service(s), replicate data in object storage, etc. Common sense stuff, if that's something that's of concern.

Personally, I've seen "vendor lock-in" weaponized as an excuse for a lot of costly NIH bullshit. It's painful to reflect back on a project that could have involved literally a tenth of the time and pain it ended up taking because of that one choice alone.