We are talking about a case when maintainer is unavailable to do the work: what would happen if this was a proprietary dependency and the maintainer is gone (eg. bankrupt, moved on, incapacitated...)?

There is nothing unusual about this, businesses face this all the time, the only difference is that you do have some agency with FOSS.

What's the alternative when it is not FOSS? Eg. build it yourself from scratch (and maintain it too), or move to a competing product.