The thing with Froome and Nibali is, Froome definitely didn't max out his career and Nibali, talent wise, just wasn't generationaly great.
Froome had his breakthrough aged 26, had to spend the first two years of his prime as a domestique, crashed out of the Tour in 2014 and got his years of tdf contention cut short by a crash. Despite being such a late bloomer, without those factors hampering his career Froome could have won as many tours as Armstrong, possibly even more, so 4 Tours (or the 5 of Mercx for that matter) are clearly not the maximum you can achieve in modern cycling.
Nibali on the other hand, with the exception of part of 2013 and 2014, never was a dominant force. He fulfills the requirement of multiple gt and multiple monument winner, yet is there really anyone thinking "there can't possibly be anyone as good at what he does as Nibali again"? Nibali is often seen as the guy who has that sort of palmares despite his talent not because of it (although that sounds a lot harsher than it is. Nibali is still clearly among the most talented riders in the peloton).
Sure, getting Mercx palmares isn't realistic nowadays, but can I see someone breaking the tdf and gt win records while winning a few monuments along the way? 100% yes. If anything those gt records are some of the major sports records that seem most breakable. Pretty sure that after the 2010 Tour there were more people thinking Contador will get to 11 gt wins, than there were people thinking he won't.