The problem is that players of different eras, rightly or wrongly, are always compared to each other. This metric, how many majors they won, will always be used to compare them (rightly or wrongly). If you give the 70s and 80s extra majors it will cause huge problems in comparing, not just to the players of today, but to the greats of previous decades, like Laver and Budge.
And you can even set aside the problem of inter-era comparisons. I'm not even that big on inter-era comparisons. Let's set it aside completely. There is still a huge problem here: tennis fans will often not know what they are saying to each other. If I hear that X player has Y number of majors, and the number is something radically different from what I usually see, then I will have a whole set of questions. I will be asking, "Are you counting 3 majors per year? or 4? or 5? or 6?" That's hardly an acceptable range of uncertainty. If major-counting can be that flexible, I can assure you, I will ignore any major-count I see, unless I know what's being included in it.
Because you win majors - no mather how many they are- in the context of your era, then we should pick up the true greats of each era and compare them besed ALWAYS on their dominance of an era.Not just a matter of counting this and that major and then use them to compare inter eras players ( as you said, it wouldnīt be unfair, but it would be also very unfair not to count WCT/Masters for 1970-1989).Because, eraīs strengths are so different that winning 5 majors in a very strong era may be harder to achieve than 10 in a weak era.