Deftones are not that expensive. Vilar de Mouros could book them and it's a much smaller festival. They were on the line-up for North Music Festival 2020, also a very small festival.
Can't speak to the Stones, but compared to Macca I'm pretty sure Elton was an absolute nother level. Purely anecdotal, but we were up at the very top in what used to be Row Mead by the Pylon for Elton and even up there it was absolute standing room only, Macca on the other hand we were about 75 yards in front of the tree and, whilst by no means empty, it was honestly pretty roomy for each group.
Will say though that that MacCartney set suffered from particularly extreme crowd crust, as when I went to the loo it was seriously rammed around the paths, with things "opening out" about 10 yards into the field
Re the quality of the sound I’m with you, it’s definitely got quieter in general and in some cases noticeably worse (e.g. WH). You really shouldn’t be able to hear people having a conversation 10 rows back at a large stage. Shambala had a new venue this year, possibly the best sound I’ve heard at a festival and they’ve got nowhere near the budget of Glastonbury.
l still feel skeptical about them having the 3 biggest pop starts of the moment and then also have Deftones and nice undercard. It’s a bit of a push both in money and capabilities for a festival like Primavera Porto. If they end up having the 3 pop girls plus Deftones plus Clairo, Twigs, Fontaines and so on, then what is the point of going to Barcelona for such a high price (350€ right now) when you can see the biggest names in Porto and most of the nice undercard? Idk. But then again, would not surprise me if they did it, it’s true that last year they had Lana and SZA and those are huge artists.