Hearthpwn

I don't know what world you've lived in but it was never any different or even better than now. Naxxramas was not a "mini-expansion" or even "mid-expansion", it was the first expansion ever, and it was released in the exact same way as new expansions these days, 4 months after the official launch, and 4 months before GvG (as the first "big" expansion) came out. Ever since, it was always 4 months until the next release of new cards. But unlike today, balance patches were rare and took much longer.

The main difference was that adventures were released in wings, while expansion come out as a whole (and have roughly 3 times as many cards). And as I recall, a lot of people specifically didn't like adventures because the cards slowly came out, which also meant that you had to buy all wings, even if you only wanted the cards of a specific one.

As for the monotony, I already mentioned that balance patches are a LOT more frequent and impactful these days. We currently can expact 2 or 3 balance changes during the 4 month period, with some decks dying entirely or matchup spreads changing drastically. That's a whole lot more changes than what followed the release of Karazhan, for example. And unlike in the old days, things like Fire Festival or Hollow's End at least provide a small amount of variety. It's not much, but I remember the days when precisiely zilch happened between expansions.

In other words, the "monotony" you speak of is mostly your perception. Of course you can feel this way, and suggest for things to improve, but it's not like today is any worse than 2 or 3 years ago, and in some regards objectively better.

About popularity:

The game isn't "dying" (if it really is at all) only because the developers don't care anymore as much as they used to. It's simply a 6 year old game that fails to be as appealing as other formats, specifically for the young/adolescent audience, like League of Legends or Fortnite are. And some other card games also take some of the spotlight that Hearthstone used to have.

There was a time where Hearthstone was more popular, but it's hard to say why. Maybe because it was new, maybe because the previous associated players/streamers were more popular (quite a few have quit, some even died), maybe because the game was perceived as better. But if the game was better, you can only vaguely point out why. The differences between the game back then and now are mostly subtle. The deck costs have gone up significantly (with several playing in the league of former "Wallet Warrior"), but few people seem to bother. Some people like to complain about "powercreep", but it really only matters within a specific metagame. And as far as I remember, Secret Paladin (for instance) wasn't exactly seen as a fair and fun deck to play against at the time either. Or Freeze Mage, for that matter, which also was the prime example for the game not changing enough, because Ice Block kept that deck alive, regardless of the original Standard rotations. Then people started complaining that Evergreen sets were way too influential, and no other set has been trashed as much as Classic and Basic since.

Likewise, it's difficult to say exactly why the player- and viewerbase is dwindling, but it's not a new thing either. It's on a decline at least since 2017/2018, with Blizzard's MAUs going from roughtly 40-45m to something around 30 right now (not entirely sure, since I haven't checked reports in a while). This is for all of Blizzard, but it's reasonable to assume that Hearthstone's playerbase isn't unaffected by these developments. Again, you can say that HS could do more to remain popular, but it would be unfair to say that it's not doing as much as it used to, because really, the devs were never as productive and pushing for change than they are now. And it's a different question what policy could help with that. Just my personal opinion, but I think anything making the game more expensive (like increasing the number of expansions) is not going to help with that.

A large part of Hearthstone's current popularity comes from Battlegrounds. Many streamers switched to Battlegrounds, partially because they like it more, and I have to assume that a sufficient part of their audience likes it better as well. The mere introduction of Battlegrounds happened last year and came out of the blue. Nobody uninvolved really expected that Hearthstone would even introduce a new game mode, much less one that is gaining this kind of attention. And it is, even more shocking, the least expensive mode to properly participate in (but also the least rewarding, save for PvE stuff).

Of course, it's possible that Hearthstone will eventually fall into obscurity, or that Blizzard pulls the plug out of the competitive scene like they did with Heroes of the Storm. But for the time being, at least, it doesn't look that way. Not that we'd know, but Blizzard moving on to another pet project doesn't seem too likely right now, given some of the controversies surrounding both Blizzard itself and live service games (and lootboxes in particular) in general. When it comes to cash cows, even Hearthstone is (and was always) far behind other mobile titles in Activision's possession, particularly the King games.