These days, it's not really up to a company whether it wants to be fabless or an IDM. Apart from the several billion dollars needed to build a competitive fab, you'll also need to contend with the 20+-year head start existing IDMs and foundries have on you in terms of experience and expertise. As far as I can tell, there are too many moving parts in a fab for a new company to simply poach a few key people and be competitive quickly. I can't think of anyone apart from Apple or a nation-state that has enough money to wait around while their new division learns how to build chips.
That aside, assuming a fab better than TSMC's showed up on your doorstep today, you're not out of the woods in terms of being dependent on suppliers. On the contrary, designing fab tools is also a very capital-intensive business and tends towards an oligopoly, so the TSMCs and Intels of the world are just as dependent on tool vendors as NVIDIA is on TSMC. The same holds for the very specialized materials and consumables fabs use, like the glass to make mask sets.
Most companies only make a small fraction of the tools you need for a full fab; for examples of heavy hitters in key areas like lithography, chemical mechanical polishing, and ion implant, see ASML, Nikon, Canon, Applied Materials. For any given tool, there are usually 1 to 3 major vendors.
• The ex-CEO was criticized for selling their mobile business to Qualcomm just before the Apple+Android explosion
• AMD spun off GloFo due to immediate liquidity problems but maintained a stake. Recently they have sold their stake, so if AMD is successful at being design-only, their actions are justified. But it would seem they are dwindling - i.e. Bulldozer is hardly a home run, and lots of sources are saying the move to automated layout of their transistors is a significant part of the problem - so being design-only could be seen as a mistake.
Obligatory: Intel uses extensive auto-layout and auto-analysis of designs; they seem to have the advantage in this area by merging automatic synthesis with focused optimizations by engineers.
