True to an extent. If player A can run twice as fast as player B and is twice as strong, and 2/3 as good with the ball, 9 times out of 10, Player A will win. It's still an athletic competition. So, to me, it's likely that Ronaldo and Maradona are, when not accounting for era, the 'best footballers ever'.
Accounting for era is where we can assume things like nutrition and fitness regimes are equal, and they are far less likely to be the best ever. But their is still the question of tactics. Would Messi have shined in Dixie's Everton? Would Dixie have shined in Messi's Barca? (I have no idea, as I'm not old enough to have watched Dixie, I'm just assuming the answers are no because of how many he scored with his head).
I actually agree that you can compare footballers more easily across generations, not because of the equipment being minimal, but because of the lack of good statistics. Football has yet to be numerically quantified very well, which means the physical attributes don't skew the numbers in a significant way and lead the eye astray. Of course the 'eye test' comes with it's own failings, such as the tendency to forget the mundane and ugly in favor of the spectacular.
I have no idea if Messi is the best ever when accounting for differences in era. Probably not, just because probability doesn't favor it, but I don't know. I do think the 'must win a World Cup' argument is nonsense. I also think that people tend to gloss over flaws from past players.
We'll never know. Because we'll never have a perfectly quantifiable objective statistical measure that eliminates all variance and ranks players perfectly.