Just a minor correction: the 100$ one time deposit cannot be reclaimed manually. Instead, it gets automatically returned once your game hits 1000$ in sales.

partner.steamgames.com/doc/gettingstarted/appfee

The purpose of this fee is to block low effort automated scam games from misusing the shop.

Many successful indie devs have voiced that the 30% is actually impossible to beat for them if they tried other distribution approaches. Some even closed down their existing alternatives including self hosted shops which would grant them 100%, simply because the overhead costs ruin the percentage for them, plus a whole lot of time and effort that have to go into maintaining that.

Yes, steam has a very strong monopoly position on the games distribution market. That is problematic for all the usual reasons with monopolies. What makes steam unique is that the company behind it, Valve, has demonstrated in all their efforts that maximizing short term profits is not necessarily their prime directive. This can obviously change at any time, so being wary is always good, but convenience is simply extremely attractive to everyone involved, devs and customers alike.