^The issue with that assumption is that we're using today's metrics to define the quality of past players. This is regarding human chess, of course - it's unfortunately indisputable that engines are a good judge of chess *quality*.
But if you have an era where players have a certain style, then to get best results against them, you should play a certain way as well - if players played much more solidly, you change to adapt to that.
How would an expert today do against Staunton, given some sort of time machine? My guess is not too well.
Choose any 50 consecutive games that Staunton played and analyse them without a computer. You will quickly find that his play was weaker than today's expert strength players. And this is a guy who considered himself the best in the world. Blackburne played some brilliant games. So have most experts. But he also played some howlers. The mark of skill isn't the best games played, it isn't the worst games played, it's how consistently you can play close to your best.
Wesley So currently has a 62 game unbeaten streak. None of the players from the 19th Century were as consistently good as the best players from the 20th Century, or the 21st Century.
I have favorite athletes, but I know that Jesse Owens' best wasn't as good as Usain Bolt's. Mark Spitz wasn't faster than Michael Phelps. Bobby Fischer wasn't better than Garry Kasparov. And Paul Morphy, for all his brilliance, wasn't better than today's GMs. Sure, he was dangerous in open positions. But chess has progressed since Morphy's day.