The consumers in the market for GTX 1080ti class GPUs is already in the minority even for DIY enthusiasts.I'd say the DIY market is the biggest though. You're not going to fit a 1080 TI inside a Dell or HP or other clone computer lol and even if you somehow get it to fit the cheap 150w PSU they put in those things is not going to handle it. The type of person that buys a high end graphics card is typically the type of person that builds their own machine. Or the type of person that has more money than brains and buys an Alienware. But those people usually just buy a console.
All the major OEMs have already been developing higher end gaming specific lines due to the shift in changing market conditions. Dell's Alienware is not the only line catering to this. Even looking at what were traditionally PC DIY companies such as MSI you may have noticed over the last years they are expanding and moving more into prebuilts, AIOs, gaming laptops, and etc. A company like Asus isn't even thought of anymore in the same way in terms of their product mix compared to 10-15 years ago.
Smaller system builders are also now setting aside cards specifically for their in house builds.
Not to mention the growth of the gaming laptop market.
The above have access to the supply channel at higher level than DIY consumers. HP certainly is not paying the current end retail prices for GTX 1080ti's going into their Omen systems.
Only a small portion of potential customers are dead set on being DIY. If we have a prolonged shift with higher DIY prices, not just GPUs but other components as well, then more consumers will simply switch to things like boutique, prebuilts or laptops. The current situation will do more to hasten the further demise of the DIY market, particularly in the mainstream, as opposed to any impact on PC gaming as a whole.