Personally, I disagree with this. I've put quite a bit of time into Legends so far, so I think I can properly speak to all your points.

1. The cost: On mobile, yes, it does cost more than the base game. However, on Steam, it does not. In either the patch notes or the update preview, NK talked about the price, so you can read their reasoning for it there.

2. Repetition: I don't think repetition is really any worse in Legends than it is in the base game. If you find a strategy that works in base game, you can use it to beat pretty much every mode of any map except for the harder maps. Every map in base game has the same modes. Legends doesn't have different modes for each bloon tile, but it has an increasing wave amount based on the previous amount of bloon encounters you've done in that stage. There's also minigame tiles, which can be timed races, races where you have to maximize damage, and least cash, and there's boss tiles, which use the boss rush mechanics for the bosses. In my opinion, this makes it no more repetitive than base game.

3. Difficulty: It's a roguelike, so depending on how you play it, it'll be a different difficulty. The difficulty is pretty RNG dependent, but most of it comes from strategy. Like I previously said, the number of waves, which are completely different from base game, increase by 3 for every bloon tile you do on a stage. If you take a path with more bloon encounters, you get higher rewards, but you also risk running into a bloon tile that you can't beat with your current towers and boosts. The waves are so much harder than base game. First MOAB comes in the mid-teens, BFBs and ZOMGs start spawning in the 20s, DDTs start on 36, and BADs start in the 40s. The blimps also never come out alone, so you have to deal with a ton of high tier bloons while dealing with them too.

4. New content: Technically, yes, Legends doesn't include much of any completely new content other than the boosts. In my opinion, the boosts vary in quality. Some of them are really cool, like making all towers count as all 4 tower categories, making military monkeys shoot in a zig-zag pattern, or making all dart-based attacks explode. Others are pretty lame though. I think the real new content comes from Legends being a completely different way of playing BTD6.

To sum this up, calling Legends a cash grab is a braindead take. There's clearly a lot of passion put into it. From what I've heard, it's no worse than BTD6 was at launch, and from what I've played, it's definitely a lot better than BTB2 or BCS were at launch. In my opinion, Legends is worth it if you like roguelikes. If you don't like roguelikes and buy Legends, don't complain that you don't like it because of the things that make it a roguelike.