I have already acknowledged what you said as potentially true, but not necessarily so. Ryan Giggs, George Weah, Eric Cantona, George Best, Alfredo DeStefano, Jari Litmanen, Ian Rush, Bern Schuster, Gunter Netzer, Abedi Pele, Anthony Yeboa etc. As a matter of fact he opposite is true, some players have played in a WC but did not shine at all, but still considered great Van Niselroy, Dejan Savićević and Ibrahimovic come to mind
The fact is, a World cup is maximum 6 games every 4 years, (18-20 on average if you count qualifying, but no one ever remembers how you performed then if you're a big team), same for European cup. A football season is 38 league games, maybe 10 cup matches, max 12 European matches. So the average high level footballer plays 400-500 odd matches in a 8 year career at the highest level, of which maybe 80 competitive ones with his national team.
He has exactly 12 of those to write himself into World Cup greatness.
With the World Cup, you can't tell yourself, I will make it up next match, in the return leg, or even next year. You have no guarantee to be around in four years, between coaching changes, injuries, drop in form, old age, non qualification
What makes the great ones great is that they seize that moment - that very small window of opportunity. That doesn't change the fact that they also need to prove is no fluke, and they do that day in and day out with their clubs as well. But at the end of the day, is all about those who could rise up to the occasion.
Most of the modern players you call there exempted from the WC = greatness rule for the simple reason that they played for national teams that didn't have a snowball chance in hell of qualifying let alone winning a World Cup. Cantona is a notable exception, but I personally think that while he may have been a great footballer, he wasn't a great player, in the sense that football is also about a team attitude, that he didn't have for most of his career, and only when he had Fergie to kinda keep him in check.
I would never use the word 'great' to describe Ibrahimovic, certainly not at this stage of his career. I can't even tell you what Savicevic did to even come close to meritting that title. These days we hand out labels like 'genius', 'great', 'master' and 'legend' like bread on its expiry date at soup kitchens...
I watching football seriously since the wc in the usa and the only players since then i, personally, would ever call great are ronaldo (the original), zidane and (maybe) rivaldo. Giggs might get a bligh because Wales is a crappy team and he do everything he need to do otherwise. Call me severe...
Of course this is off topic in so many ways...