Simple, by taking the character back, when it's at an all time high, and creating their own universe. You underestimate the greediness of executives. Like I said, they are rumored to still wanna do a venom movie now. Make no mistake about it, they would want to do SS and all the other crap we read before.
Think it from their perspective, "we got a brand new trilogy finished now, Spidey movies are averaging $1B or whatever, and the character is at an all time high. Let's take full control of the character and go on our own. We will now branch out and create our own superhero cinematic universe."
Plus from a legal perspective, the contract would have to be extended and/or amended after every 3 years or trilogy. This can bring more greediness and since Marvel gave up so much for a shared spiderman deal to begin with, Sony will feel like they have the upper hand or take the character back as described above. Nothing is extended in perpetuity unless it's already on paper. Especially when we are talking about millions and billions of dollars. One recent example, James Bond, made the most money EVER under Sony. There is a rumored bidding war for the franchise right now.
This deal is good for short term and a mess for long term. The publicly available information on the deal greatly benefit Sony and are just okay for Marvel.