The issue with "go start your own instance" is not everyone has the technical know-how or financial resources to set up and host their own website running a ruby-on-rails webapp. Also, not everyone is confident in their own abilities to moderate a community and be a respectable leader. None of that invalidates someone's concerns about where they are and what they need.

That said: having more instances is absolutely a good idea and will really help a lot of mastodon issues