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