that's the thing: "lots of other instances" doesn't mean much depending on what you are looking for. mastodon.social has over 400k users, and federate with most of the 10k+ user instances. So, having lots of 1k user instances blocking it still doesn't matter as much.
And the second thing is the type of instances that are blocking mastodon.social: based on what I usually see going on on mastodon and lemmy, they are probably instances focusing heavily of LGBTQ+ or other marginalized communities. So unless that's the most important thing to you, you won't miss it.
quick note: those communities can be REALLY annoying sometimes. I remember seeing people mentioned that they got jumped in because they posted food pics without content warning, in an instance about.... food and cooking. So, if instances like this are blocking mastodon.social... go ahead
I went for fosstodon.org, as I think this suits my interests well enough, but there are other general or broad topic instances out there.