Cutting edge foundaries and the machines inside them are the most advanced technology mankind currently possess, the actual process is literal scifi shit. It would take decades (plural) for someone starting at nothing to make the chips inside your toaster. Even the competing companies that were doing it for decades (Glofo and Intel) can't do it. China most definitely can't just "make their own" on a whim? LOL
It would not be starting at nothing since the principle is known, I think I remember some ASML guy saying basically anyone can copy it- and China does not care much about patent laws anyway, if there are some -, you "just" need to make those precise machines. A ban is certainly an incentive to put more pressure behind it, more than just the possibility of profits. If you kinda need it, it's like wartime industry, ramping up the necessary stuff and making usually huge progress.
The timeline is the question. The investment might have been too much for the retired players, but if China needs it and is actually forced to, it might happen sooner than one can expect. Hardly tomorrow, but 5, 10 years are a long ass time to get something up and running, especially when SMIC has already the foot in the door with a worse process.
"Decades plural" seems like equating the cheap garbage plastic toys effort with Chinas actual brightest and what they can do. I mean decades ago, bascically only two countries bothered with space programs, ESA followed, and China, Brazil and India too, even private companies shoot stuff now into orbit, because literal rocket science is not really anymore something that is almost magical, but just very humanly possible today, if tackled properly.