Switching costs to individual decision makers are perceived as high, justifiably.
Imagine you're a decision maker, and you have the chance to either (a) stay on with Oracle or (b) go with some set of startups that provide everything Oracle does. Which is, to be clear, a whole fucking lot, even if it does them badly.
Best case scenario (which is hard to pull off), there are no hiccups and you end up saving your company millions or tens millions of dollars per year. You get some reward for it, but definitely not one ongoing or even close to commensurate with the value you just provided.
Most likely, you run into significant hiccups. Maybe one of the startups you chose was a bit optimistic for what necessary features they could deliver in time for the Oracle switchoff. Maybe it's unreliable and all the users hate it. Maybe it takes way more time than expected to retrain the staff to use all the new things. Maybe Oracle drags its feet about helping you stop being their customer. Individually these are all perhaps evitable, but you're likely to run into some issues. Then you have egg on your head. And even if Oracle was just as bad in all those things, because you were the one who rocked the boat, the blame is on you, instead of "well, that's just the state of life."
You might say, "well, why not do things piecemeal?" And you can try that. Indeed, you are ethically obligated to employees and shareholders to do that instead of introducing huge risk to everyone because of an understandable anti-Oracle vendetta. But the way Oracle structures its sales and legal strategies makes it much less financially beneficial to you to do that than to cut them off whole hog. So you scale down the risk, but the potential benefit scales down even more so. And of course, who knows if Oracle won't just acquire the company you switched to a year or so down the road. And then you're stuck where you were originally.
I think, practically, once Oracle has its claws in your company, you're stuck with the parasite. The only solution is to categorically cut off Oracle from consideration for any vendor-supplied solutions. If it acquires one of your vendors, you make it a priority to transition off of it as soon as possible. If there are no other options, you start a new company to fill that niche.