The danger for Pogačar is that if and when he catches and passes Merckx, some won’t be ready for him to be called the GOAT. When Federer passed Sampras for career slams in 2009 (15 vs 14), he was immediately anointed the greatest. Even before they people were calling him the greatest. When Djokovic caught and passed Federer and Nadal, people weren’t and some still haven’t accepted him as the greatest.
My thinking is that the same will happen with Tadej.
The thing with Tennis is that though fitness, tactics, technology, speed and competition levels etc. have changed, the sport is still fundamentally the same and it is a lot easier to compare through eras than many others. It hasn't changed its characteristics as much as, say, football, where the things defenders could do, the quality of pitches, the concentration of talents into bigger international leagues, the Bosman ruling, and technology of boots, pads and balls have changed the sport - but it is still fundamentally the same. Nevertheless, a lot of people still balk at suggesting any of today's players are greater than Pelé, even if they've surpassed many of his achievements, simply because many of those achievements were not available to him at his time, and the nature of the game when he played made him stand out more than it would be possible for anybody to today, even with the Christ-like cults of Messi and Ronaldo that have been at each others' throats for the last decade.
Cycling is beholden to a number of outside factors as well; not only have changes in professionalism fundamentally changed the sport, tactics have been revolutionised several times, the road infrastructure that underpins the racing has developed and the technology of the bikes (and the sports science, Clinic and otherwise) has enabled far higher speeds and a far wider range of geographical options for the courses, as well as producing significant changes to the calendar, so the sport as raced today would be unrecognisable to Merckx racing at his time, and putting a finger on what Merckx would have achieved if racing today would be nigh on impossible. Likewise if Pogačar came along in Merckx's day, he'd probably have been 23-24 before he was even allowed to race in the West and would have been fighting against a well-oiled Soviet machine in the Ostbloc races, and his talent would likely not have been nurtured to the same extent, so it's impossible to tell what he may have been capable of.
It's like saying, right, who's the better driver, Fangio or Hamilton? Fangio won 5 titles, and Hamilton has more, but the sport is so fundamentally different you just can't compare them. Hamilton won many of his titles thanks to team orders, a superior car and passing aids that meant that even when he had to come from deep in the field, opposition would just let him through because they weren't expecting to race him for position; the races were all held on mostly very modern, safe and sterile autodromes or "street circuits" that are street circuits in name only, billiard-table smooth and where driver aids and computer telemetry mean much of the race can be stage managed from the pits. However, Fangio may have been competing in an era when death was a constant spectre and courses ranged from twisty two-way roads to 6 mile straights and courses were lined with trees and ditches, but he also had the opportunity to commandeer a teammate's car if his broke down, had to be good over a much smaller number of races (and only in the ones that counted as there were many non-championship rounds back then) and in some fields where very few drivers were ever going to be competitive and the overall standard of driving from the best to the worst driver on the grid was colossally higher than today.