Dynasties are boring only if they are built on some kind of advantage in the sport that pushes some teams to the top and eliminates others from any reasonable chance at contending. MLB was like that back in the middle of the last century, when teams from the larger markets dominated the game and many small-market teams were no better than minor league franchises.
I don't think a dynasty built on a solid organization, great personnel decisions, and a great coaching staff hurts a sport at all. A lot of hockey fans will tell you that the first 25+ years of the post-expansion era in hockey (roughly 1967-1994) was the absolute heyday of the National Hockey League -- and that era was dominated by dynasties.
The era from the 1967-68 season to the 1993-94 season covers 27 seasons, and there were only two Stanley Cup champions in that era who only won a single title:
Montreal (10)
Edmonton (5)
NY Islanders (4)
Boston (2)
Philadelphia (2)
Pittsburgh (2)
Calgary (1)
NY Rangers (1)
That era saw some of the greatest players the game has ever seen, and included team and individual performances that may never be matched in the NHL again. Notice that half of the teams on that list would be considered "small market" teams today, and most of those small-market teams faced financial troubles later on that threatened their existence.
I agree with your assessment of the NBA, but that organization has a whole different problem. Basketball is a simple game with a smaller cohort of players than any other sport, and it's easy for a team with two great players or three very good players to dominate the game. I don't know what has changed in the NBA in recent years (I haven't watched a basketball game in years), but as recently as the 1980s there were a lot of exciting playoff series in the early rounds even though the Celtics, Lakers and Pistons seemed to win most of the titles back then.
The problem with dynasties is they disengage the other fans. Sports fandom is built on the lie of “maybe this year”. And the sooner that lie gets shattered the sooner fans disengage. Your NHL list is a great example, while that era saw great players and great play not very many people saw it. That was the era when the NHL lost TV contract after TV contract eventually going a decade without an over the air contract. It took until the Comcast/NBC merger to really fix the TV revenue damage done in that era.
That’s the balance sports leagues need to find. A few teams capable of performing the game at the highest level makes for beauty in the sport (at least when those teams face each other). But it’s bad for revenues. Revenue comes from as many fans being engaged in the season as long as possible. That era of the NHL was enjoyed by fans of the Habs and general “pure” hockey fans (with some windows of engagement from Isles and Oilers fans). But fans that didn’t look beyond their team lost interest quickly during the season. The current NHL world where most of the league is in the hunt for the playoffs until the last month and anybody that makes the playoff has a shot is much better for fan engagement.
I almost hate to admit it, because he was such a putz. But you can make the argument that no commissioner in history ever corrected problems in his sport as well as Bud Selig did.
Between the wild card, CBA changes, and revenue sharing, MLB has probably never had a moment in its history where so many teams can reasonably expect to contend in a short period of time.
The Pirates are just mediocre and not God awful even after Andrew McCutcheon was stolen by aliens and replaced with some kind of strange duplicate AND their third baseman got arrested as some kind of international human trafficker AND their current best player got busted for being hopped up on goofballs.
It’s been a fun year for the Buccos......