Sony will never let go of the King of Marvel and arguably the superhero world.
Yeah. They won't ever let go of him.
But that doesn't mean they aren't willing to "share" the character, and work with Marvel.
Sony's Spider-man being tied to the MCU is beneficial for both sides.
Marvel gets the benefit of their flagship character being tied to and appearing their flagship film series and Sony gets the benefit of the extra exposure their spidey films get by existing in the same universe as the, now, very popular characters Marvel features in their MCU films.
Also, by having Sony still work on the solo films, while giving Marvel the right to incorporate him into their universe, w/ potential crossovers, it saves Marvel the large amount of time, money, and effort they would otherwise have to put into making the Spider-Man films.
One of the main reasons Marvel is fine with Spidey being with Sony is because he needs waaay more focus than they can give him with their MCU slate. Because with the way Marvel is heading with the MCU, they wouldn't be able to give the character regular released films like he should and they would want him to have. No one character gets more focus in the MCU than the others. Not even Iron Man, who will be noticeably absent from Phase 3 outside of The Avengers 3.