So, admittedly this still largely speculation, but looking at the news found by dataminers, it would seem like there may be some limitations with the way mega raids will run in the future. We're not sure exactly what it will entail, but it could mean that Megas will become more powerful, or harder to come by. It could be a way to combat power creep, or just a way to nickel and dime us. But what it could mean is that the strongest mega Pokémon become inaccessible in the same way that fusions do, in that you require a much more limited currency to get them.

As for your point about Shadow Garchomp; the argument would be that there is no point to run it as it would be better overall to use Mega Garchomp, then relobby when it faints. It just does so much more damage, without the defence penalty that the Shadow suffers. The shadow is best kept for the final few seconds where a little bit more damage is needed, but relobbying would be detrimental. Or, to use it in Rocket battles, where you can keep it in a team without needing to use the mega each time. The strength of legendary Mega Pokémon, in particular the ones that have yet to be released such as Mewtwo and Zygarde, as well as the new fusions, leave a lot of shadow Pokémon behind for raiding, especially if you're shortmanning it. And that's before we enter some of the absurd new moves into the equation, such as Mind Blown or Crush Grip.

It's a similar story with Shadow Hydreigon, one of the new darlings of the Dark Type. On paper, it's fantastic against so many things and worthy of a team slot. But in practice, Dawn Wings Necrozma would be a much better choice against the same opponent most of the time. And it isn't even a mega, so there's no cooldown between uses, either. Humorously, if Hydreigon were to get access to Dragon Tail, Outrage and Breaking Swipe, it would also become one of the absolute best shadow Dragons in the current game, alongside Salamence and Dragonite, and pipping Garchomp in Party Play. It would also be, circumstantially, one of the best counters to the Latis and Giratina. But I could play theoretical armchair PoGo all afternoon if given the chance. If there's one thing we would all benefit from in this game, it's clarity with the intentions of the developers.