1) side loading, or however it's called, is used by less than 1-2% of global Android users (we can't be more than 50 million). Google made us a favor leaving it open after an only 24h delay. It could be much worse but now it's nothing in our eternal tinkering with developer options. Thank you from me Google.
2) GMS is a huge convenience for any app developer that needs tight control, including governments. They can secure their apps against users of any hat color. Add to that the possibility of hidden backdoors to support surveillance and Google's direct lobbying in EU. This makes it very difficult to go without it, even under the current anti-US EU direction and it will be the last that will be replaced in Europe.
3) There are various levels of "degoogling". From installing a totally open OS without or with microG, to just don't login to a Google account in stock Android. It's a spectrum but someone at the free edge will never have the same rights with one in the jailed edge.
4) Developer verif is NOT to prevent ad blocking. There is a simple and free method to block anything you like at DNS level: just select Private DNS and insert an appropriate URL for ads/trackers/porn etc, eg from controld.com. Find some other justification, like tight user control to continue sleeping with the governments or reach ultimate user surveillance with the upcoming children ident.