My argument is every single monopoly eventually becomes 'bad' and makes progress stagnate and prices rise.

Just look at Google and Amazon. Up until I'd say late-ish 00s, nearly everyone loved Google and Amazon. I remember when Google search, GMail, Maps, street view, etc. first came out. I loved them, and I felt they were so much better than the competition. Same thing with Amazon - their selection was huge, great delivery and customer service.

I still use Google and Amazon for so much, but it's usually with reluctance and distaste. Check out Google search results these days. The ENTIRE first page about half way past the fold is ads, or AMP carousels, or shit I just want to get out of my way. But I still use them because with their heft and scale no one does long tail searches as comprehensively.