Honestly? I don't see why people dislike this. Yes, Mew is on-disc DLC that costs $50 to unlock, but so what? It's 2019 now, and it was 2018 when LGPE came out. Gamers need to understand that DLC and microtransactions are the future.
$50 isn't even a lot of money. Look at fighting games, where characters cost five to ten dollars. Even the people who supposedly love fighting games will complain, "oh boo hoo i have to pay for characters when I used to get them for free". Pokemon fans may look at the $50 price tag for a single Pokemon and think it's a bit much, but let me reiterate:
A POKEMON is worth A LOT MORE than a few fighting game characters. And it's MEW! Mew is one of the coolest Pokemon ever.

In short, fuck anyone who doesn't think that being able to buy legendaries is a good thing. I hope that they allow us to buy the box legends for Sword and Shield because honestly? A true Pokemon fan should be happy to support the company they love.