Most non-trivial software projects require at least 3 months of onboarding to be fully productive. Hiring takes even in the best of cases about as long.

I would not consider that to be "easily" even for mid-tier devs.

Is it really? I feel like I can easily tell who is easy to replace or who is not in my team.

If you can't tell if you are a special snowflake or not, you can check if your company behaves you like one. Getting promotions, extra RSUs or very high bonuses should be an indicator

I think it's more like: everyone is aware how lengthy a process it is to replace them.

Everywhere I went companies had bold plans regarding hiring, but somehow never managed to realise them.

Maybe now with all the layoffs… but a year ago finding a good dev was very hard and expensive.