1. Saying "hey, you can just not list on Spotify" is naive. Unless you're a major artist, you don't have the market power to convince people not to use Spotify. Essentially every labor movement is about pooling the collective power of individuals to fight larger entrenched market owners, and that's what this boycott is about.
2. To me the main issue is not the payout percentage, but how it's divvied up. I believe this is still the case, but payouts are divvied up by averaging across all plays. But the total plays are dominated by large artists. A better deal for smaller artists is to allocate each individual subscriber's revenue based on what that subscriber listens to. For example, if I love Obscure Artist A, and 90% of my songs are Obscure Artist A, then Obscure Artist A should get 90% of my $15 or whatever subscription fee (minus Spotify's cut). But instead, Spotify says "Obscure Artist A only had .000001% of total plays, so they only get .000001% of total revenue" - it ends up being a better payout for the big names but a worse deal for all the smaller artists.