This thread has motivated me to try to analyse some of the differences between the league now compared to in previous eras of great teams, because I think we almost all accept (statistically counter-intuitively) that the 1999 United side that accumulated 79 points was a greater team than the City team of last season (100 points) or this season (98 points). It's statistically clear that the gap in quality between the top 6 and bottom 6 is greater than it's ever been in terms of the share of points. However, what's interesting about that is that at the same time, the gap between top and bottom in terms of revenue is actually slightly narrowing. I would have expected that the narrower the revenue share, the narrower the points share, but that's not the case.
Accurate revenue data is quite hard to find on a club by club basis, the earliest full set I can find is from the 2011-12 season, when United had the highest revenue at £320m, about 6 times higher than Wigan, who had the league's lowest revenue at £53m. In terms of points share, that season the top 6 earned 446 points, about 2.1 times more points than the bottom 6 earned (210 points). The most recent full revenue set I can find is from the 2016-17 season, when United had the highest revenue at £581m, about 5 times higher than Hull, who had the league's lowest revenue at £117m. In terms of points share, that season the top 6 earned 477 points, about 2.3 times more points than the bottom 6 earned (207 points). So the revenue gap has shrunk between 2012 and 2017, but the points gap has grown.
This season's revenue data obviously isn't available yet, but we can already see that this season the top 6 earned 474 points, about 2.5 times more points than the bottom 6 earned (191 points). So that points share gap seems to be continuing to rise, even since 2017. And it's certainly a lot higher than in the 1998-99 season (the season of the greatest PL side), when the top 6 earned 411 points, only 1.8 times more points than the bottom 6 earned (226 points).
It'd be interesting to try to explain the disconnect in terms of revenue share vs points share since 2012. I guess on the one hand it's good to know that the Premier League's overall revenue model, particularly the equitable broadcast revenue share is managing to keep things more even than in other leagues, although that is set to change a little bit (for the worse) with the new calculation for how international broadcast revenue is shared. But I think it's bad for the league in general, in a competitive sense, that the points share gap between top and bottom is growing. The Premier League loves to market itself on the notion than anybody can beat anybody, but it seems like that marketing spiel is drifting further and further away from reality.
So why are the top clubs earning proportionally more points? This is all conjecture on my part, but I'd suggest a few things:
1) Year on year accumulation of greater revenue: this has allowed the top clubs to continually spend more each year to recruit the best players, and over time that has meant that the gap in quality has just continued to grow (and will continue to grow). This trend is made worse when the identity of the top 6 is more set in stone, as it appears now;
2) Squad sizes: over the years, bench size has gradually increased, which has allowed the top clubs to maintain larger squads because it's possible to keep more players happy, more of the time when there's more spots available in the matchday squad. Greater revenue obviously also allows the top clubs to pay the wages of a larger squad;
3) The lure of the Champions League: this is much more conjecture, but it feels like the top players these days are much more focused on playing in the CL every year, and this has led to a stockpiling of the top players at the top clubs. Top players simply won't accept playing for a long period of time in a non-CL team. You see top players using this explanation for a move quite regularly (e.g. Zaha's recent comments about potentially moving on from Palace).
And finally, does any of this matter for a comparison between e.g. United's 1999 team vs City's 2018 or 2019 team? I think it's fair to say that it was more challenging to win the league in 1999 because the quality gap between the top and the bottom teams was smaller, which makes United's treble all the more remarkable. But I don't think it's possible to say with any certainty whether United's 1999 team would beat City's 2019 team, and nor does it really matter. Different era, different game, both great teams. But United's 1999 team achieved more, which is why they are rightly considered the greater team.