I wonder how big a downside an x86 monopoly would actually be these days (an M4 MacBook being the best perf/watt way to run x86 apps today as it is) and how that compares to the downsides of not allowing x86 to consolidate efforts against rising competition from ARM CPUs.

The problem with the "use the GPU in a SoC" proposition is everyone that makes the rest of a SoC also already has a GPU for it. Often better than what Intel can offer in terms of perf/die space or perf/watt. These SoC solutions tend to coalesce around tile based designs which keep memory bandwidth and power needs down compared to the traditional desktop IMR designs Intel has.