The bot solution is not necessarily a bad one, assuming you're okay with having the arena as being part of a PvE/PvP hybrid feature. Personally, I'm fine with leaving platinum as the whale playground where they can compete over who can give Plarium the most money all they want, as long as any tiers required for progression is more forgiving and more of a fun experience trying to fight different team configurations with your own. For that particular purpose, bot teams are probably better than player teams, as it will help to mix up a stale meta. If you insist on making a feature part of your overall progression though, then you should also ensure that everyone can get there with a reasonable amount of effort.
The issue is that the arena is not just a PvP ranking system (as bad as it is), it's also a component of your PvE experience and not a small one either. Without the arena, your options in the forge are restricted. Unless you get get to silver, you can't get great hall bonuses above 3, and even that is painfully slow when you need a minimum of 250 bronze 3/4 wins to get a single one to level 3. It prohibits you from participating in tournaments involving the arena, for example I would need to win 186 arena matches just to get in the top 10 of the current one (which unless I'm mistaken would translate to about 83 wins per day). Progress missions are, as mentioned many times before, hilariously out of sync with reality. There are daily quests that depend on arena participation. I'm sometimes struggling with the one to gather 30 materials.
So on one hand, you have the game tell you basically from the start that here is something that you should participate in, practically taunting you and partially blocking progress if you dont, and on the other hand you have the harsh reality pushing back and telling you to go screw yourself. Let's have a look at another similar feature; the tag team arena. If anything, that's even worse than the classic one, but do people complain about it? Well, okay, yes to an extent, but nowhere near the same. Why not? Well, presumably because the only thing you lose out on by skipping tag team is the bazar. I don't think it's that no one cares about the potential rewards from the bazar, I'm certainly interested in them, but rather that without any pressure from the game for you to actually do it, people are more willing to acknowledge that it's not content that they are ready for yet. Likewise the doom tower. I can't get past the scarab boss, but I don't feel any particular need to complain about it because I know I'm not there yet and nothing is telling me that I should be. On the other hand, you saw what happened when they made doom tower participation a requirement for completing the advanced quests; some people were not happy about it, because suddenly there's that external pressure to participate in something they felt they were not ready for.
I'm not sure why this seems to be a surprise to some people. It's not exactly rocket science here. If the game pushes hard to set up expectations that it then can't possibly deliver on, it's not exactly shocking that people get upset about it.
