If NVIDIA treats ARM poorly, that benefits alternative ISAs and may ultimately undermine NVIDIAs investment. If NVIDIA treats ARM well, then that benefits the platform as a whole. The reality is going to be somewhere in the middle.
Market equilibrium does not imply maximum benefit for any one interest group. "Consumers" or "end-users" are often conflated with "society".
And while people may grouse about many things, chiefly paying a perceived Nvidia tax, I don't think anyone would accuse Nvidia of being strategically incompetent.
The more likely negative outcome seems to be that Nvidia would steer future ARM development so that their CPU+GPU solutions are more performant than anything anyone else can afford to produce.
Which seems like a negative... but far from the worst negative.
TBH, I think ARM's non-GPU business isn't interesting enough to Nvidia to screw with.