You know you can just make the wing engines 50% more powerful, right?
The 737 MAX suffered a number of bad design decisions to accommodate its newer, more powerful engines. Its engines topped out at about 8% more powerful than the 737 NG engines.
You realize this is not quite how aerospace engineering works, right?
The plane which ended up being the final nail in the MD-11's coffin, the 777, didn't start development until the 90s. Of its three initial engine choices, two were derivatives of engines that were around when the trijets came to be. The initial version of that Rolls Royce engine was so late (and so unreliable) that it essentially killed the Lockheed trijet. The third option, the GE90, was the largest turbofan engine at its introduction until it was succeeded in 2020 by the GE9X.
Scaling these earlier engines up to fit an MD-11 sized twin was never an option.
Several comments, including yours, seem to have misconstrued that to mean I think the MD-11 in particular could be retrofitted into a twinjet. That's, uh, wildly mistaken, and not something I've ever claimed. I just think trijets in general are a design dead-end. Again, that doesn't mean it didn't make sense in 1960s when the DC-10 was being designed.
It is possible “to make appropriate decisions” up to a certain size. They didn’t stop making new quadjets because the design doesn’t work as well as a twin engine, but because airlines don’t need/want aircraft that large. You wouldn’t build a successor to the A380 as a twin engine.
There's a lot of other changes, of course, but more powerful wing engines let you build a bigger plane in the same kind of shape. Changes in flight rules are also significant; if twin jets can't serve all your routes, you most likely want trijets to cover the routes that can't be served by twins and don't demand a quad ... with current flight rules and current engines, twin engine covers pretty much everything.