Intel 15/16" Macbook Pros had an Intel CPU that maxed out at around 100w and dedicated AMD GPUs in a laptop that is thinner than current Apple Silicon MBPs. Razer just released a fairly thin 14" laptop that contains a 125w Nvidia GPU.
M1 Ultra is around 60w max for CPU and 100w for GPU, I believe. Don't quote me on the figures.
It's possible to make a laptop that's fairly thin and houses an M Ultra chip - especially in an 18" laptop - and have a decent thermal experience. You can think of an Ultra SoC as normal highend AMD/Intel/Nvidia laptop chips. The thermals are similar. The fans will run much more often though. But when you're not maxing the chip out, it should stay fairly cool and quiet.
I'm guessing anyone who buys an 18" Macbook Pro with an Ultra SoC is willing to put up with extra fan noise and heat. To them, they're buying a portable workstation and don't care about it being noisier/hotter.
I can see Apple eventually making a Macbook Ultra that is 18", thicker than current 16" MBPs, and contain an Ultra SoC. They seem to be moving towards the Ultra branding with the Watch Ultra, rumored iPhone Ultra, and rumored 14" iPad. It seems like Apple wants to make a lineup for people who don't care about having thin/light devices and just want more power.