Stockport always seemed quite busy (pre Covid) with passengers getting on and off London trains considering it has three trains an hour. As someone has already pointed out, it already serves an important South Manchester catchment. In London flow figures produced about 10 years ago as part of the WC Route Utilsation Study, it was the 8th busiest London flow after Manchester, Birmingham, Liverpool, Northampton, Coventry, Bham Int, Rugby, Stockport. 9th and 10th were Preston and Glasgow. Stoke did not figure in the top 10, neither did Wolverhampton. Although the passengers volumes will have increased over those 10 years, I would say the respective rankings would be pretty much the same. These rankings also most certainly do not reflect the 'on paper' size of the settlements that they actually serve.
Stoke never seems very busy with London passengers getting on and off trains there, never more than a handful or so off peak and around only 20 to 30 peak spread over two trains an hour, based on my experience and observations over many years travelling to and from Stoke over the years at different times of day and week. I guess quite a few price senstive Stoke passengers will also use the cheaper LNW/London Midland services. With an urban population of around 400k if you include Newcastle UL and other nearby settlements, plus other towns within it's drive catchment, you would expect the London trains to and from Stoke to be much busier, and certainly have figured in the top 10 London flow rankings, but it doesn't, and they're not, hence population/size etc isn't everything and lots of other factors come into play, not the least the fact that Stoke and it's surrounds are not particularly affluent, whereas much of Stockport's catchment area is.
To me it appears Stoke gets two London trains an hour purely out of convenience as it happens to be en route between London and Manchester and as you would have to crawl through it at low speed anyway, you may as well stop there and pick up or drop off a few passengers and benefit from the fact that extra frequencies will also generate an overall larger flow of passengers and revenue.