The doctrine of illusory promise makes a lot of the 'we can change the license however we want' clauses very weak, as far as contractual robustness goes. There are other similar contractual doctrines that attempt to deal with similar issues, but the issue isn't that the release valves don't exist. It's that they're old, rusted and require a lot of force to turn.
The illusory promise doctrine is very seldomly used which means there's a lot of uncertainty about whether or not it would even work. And that's if the dispute gets into the courts in the first place. Which courts it gets into is another big deal; some T&Cs put requirements that the choice of forum for disputes is a favourable one for them, or they force arbitration to be the primary dispute resolution mechanic.
That said, the illusory promise doctrine CAN and HAS demolished these type of T&Cs in the past. Sometimes they even blow up other contractor favouring terms as they go (like the aforementioned choice of forum/arbitration clauses, etc.) But why are people still putting this language into their agreements? Well...
Even if the court doesn't like that segment of the contract and they sever it, there are other ways for the contractor to indicate that the bargain they've arrived at shouldn't be disturbed - including pointing towards continued performance and a lack of notice from others, so even if you correctly note that the contract is busted, the court might say you tacitly agreed to the changed terms and they're read back in, anyways.
Civilian systems have a different strategy for dealing with these types of contracts, calling them contracts of adhesion. They aren't the product of a negotiation, so they treat them will less lenience and deference.
Common law countries often adopt this style of protection through Consumer protection laws or other similar vehicles where the rules of contract are changed to fix different types of contracts.
Basically, it's very complicated - this is just my top-of-mind snapshot on some of the issues, and that complexity creates cost, and that cost prevents oversight.