I think there’s several factors at play:
1. In general, the amount of talent in the league is always going up, as population goes up, the game expands to other countries, and the monetary incentives to try to play professional sports go up (not to mention, when we go far enough back, there was segregation).
2. While the talent in the league is going up, that does not mean that the talent at the very top end is going up. When it comes to far-right-tail outcomes, those are also just kind of random. In some timeframes, there’s just not a lot of MVP-level guys that came into the league, while in other timeframes there were a bunch. For instance, the years surrounding Jordan coming into the league were a lot stronger at the very top end than the subsequent several years surrounding when Shaq came into the league. In a league where the top few players have outsized value and one of their teams almost always wins the title, that makes a huge difference in how much a player stands out and how easy it is to win a title in certain timeframes (i.e. it’s easier to win a title in a timeframe where the guys who should be in their prime are a weak era of players at the very top end, and vice versa). I wrote about this in the following thread:
https://forums.realgm.com/boards/viewtopic.php?f=64&t=2373809.
3. While the general talent level is always going up, the league is not always increasing its size to account for that. Rather, it does it in fits and spurts with league expansion. Overall, it’s probably been pretty good about ensuring that the talent level of teams are similar over time (i.e. having fewer teams in eras where there’s less overall talent), but time periods just after an expansion or just before an expansion might be at least a little different in terms of average talent level on teams. This goes for all teams (they all have to give up talent) and what’s given up by the good teams usually is quite expendable, so it probably doesn’t make a huge difference overall in terms of difficulty of winning a title. But, at least on the margins, a time period just after an expansion might increase the relative value of top superstars even more, since there’s just a bigger gap between them and the average player in the league.
4. The ABA is its own one-time factor that needs to be accounted for. While I don’t think it was ever quite as strong as the NBA, it definitely took out a very significant portion of the talent from the NBA, making the league weaker. And this was more true as time went on, since the ABA generally got stronger as it went on. Having some of the very top stars not actually be in the NBA definitely makes it easier to win a title in the NBA.
5. Within a given overall league environment, the conferences are sometimes very different in strength. As others have said, guys like West, Magic, and LeBron benefitted quite a lot from his. And some players have been on the other side of this. In general, conference strength definitely has a massive effect on a team’s chances of making the Finals. And making the Finals is a positive, even if you lose when you get there, so that helps a player’s legacy. That said, winning the Finals is most important, and being in the weaker conference helps in that regard but not necessarily massively. After all, if you are in the weaker conference and win the Finals, then that means you beat the team that got through the harder conference. Which suggests there’s a good chance you’d have gotten through the harder conference too. That said, it doesn’t *necessarily* mean that, because the more difficult conference often results in difficult matchups in the second round or even the first round, and so there’s just more opportunities along the way to get beaten by a good team. Just because you beat the team that got through the tougher conference doesn’t mean you’d have been able to also beat the one or two additional difficult opponents you’d have had to face in order to get through that conference yourself.
An assessment of how all this affected a given player would require weighing the effect of all these factors.