A major dynamic to factor in is the linear time in which their two careers sit relative to each other. Usually the new generation sits on the shoulders of the previous generation and takes many of the advantages of sitting on those shoulders.
In Schumacher's case he arrived as a 22 year old and then revolutionised the sport in terms of driving style, fitness, technical input, discipline, and so on. It's also worth noting that today we have bullet proof reliability when in the old days these prototypes would break down contantly. Do you know who were the first team to produce bullet proof cars? It was the Schumacher era Ferrari. This is not a coincidence and it contributed greatly to his (and everybody's since including Vettel & Lewis's) career statistics.
When he finished his main career in 2006, Lewis Hamilton arrived in 2007 as a 22 year old in a formula that had transformed under the Schumacher era. Lewis and the others sit on the shoulder's of Schumacher's fitness standards, technical standards, car reliability standards, sports science standards, team cooperation standards, etc. At the end of the Lewis era in the early 2020s, the sport doesn't seem as different in all these standards compared to 2007 as it did in 2006 compared to 1991. And indeed neither does 1991 compared to 1977. Usually there is a progressive curve upwards as time goes on and generations succeed each other. There was a definite spike in the Schumacher era that looked like this if one were to visualise it:
Back to linear time. Not to get too wacky but swap Lewis and Schumacher's debuts, and imagine how each reality would have panned out, based on what we know about them. Would Lewis have revolutionised the sport from 1991 in terms of driving style, fitness, technical standards, discipline, etc? There is nothing in what we've seen to suggest he would. So what would Schumacher do if he arrived in 2007 as a 22 year old just after Lewis retires? How would he have benefited from sitting on the shoulders of the previous generation?
I put it to you, that a normal succession curve would have occurred during Lewis's 1991-2006 career, and we would have see Formula One revolution in standards take place from 2007-now.
Lewis has a massive advantage in any Schumacher/Hamilton comparison, of being the guy who came after the revolutionary finished his career. Even by 2006, other drivers like Alonso and Kimi were closer to Schumacher than the mid 1990s, because they all took up the sport science standards (electrolyte blood testing, etc), Schumacher's left foot braking driving standard, fitness standards, technical involvements etc.
I put it to you that if Schumacher arrived in Formula One in 2007 after Hamilton had just finished his career, there wouldn't even be a comparison between the two today.