What exactly are you inhibited from doing by missing CD?
You can still take gyms down with C tier attackers. You can still win raids if you get involved with a good raid group. You can still collect pokemon and send gifts and trade. Heck, most hardcore players even pack extras of the CD/A-tier mons to trade to newbies, so you aren't even 100% cut-off from getting those CD mons.
The only reason that missing CD's should be devastating is if you are actively trying to get yourself to a point where you can duo/trio different high power raid bosses as a challenge; basically: You're trying to join that upper 1% echelon of the game's best players.
If this is the case, there should be no complaints about CD's. You make it out to them, and that's it. Missing old CD mons? Trade with one of your 1% buddies who has a spare.
I have a friend who has upwards of 30 spare shadow-ball mewtwos and you know what the crazy thing is? Despite having 0 shadow ball mewtwos, I also have no reason to trade him for any of his. I have 7 Lick/Shadow ball (or soon to be shadow ball) gengars which out DPS Shadow Ball mewtwo anyway, and I have more than enough regular mewtwos to make a Psychic mewtwo squad.
I have 3 MM metagross, but other than being good gym attackers, they really aren't best in class for ANY upcoming raids.
What's my point: As a new player, if your goal is to become the very best, Niantic is ALWAYS releasing more pokemon which can compete with or downright outclass the previously existing Best In Class (BIC) mons, which means that as long as you play and grind rigorously, you can certainly still reach that 1% echelon of best players.
If instead your point is to complain that the best pokemon currently in the game can't be caught in the wild and you're mad that you can't just HAVE the best mons in the game, or that you don't like that you can't have the best mons in the game despite not wanting to put in the effort to make it out to CD's, I have no sympathy for you.
The absolute best mon/moveset combos have yet to be released in the game or look promising to be re-released. The only exceptions are Moltres as best fire attacker (which may still be ousted by BB Blaziken/Reshiram) and MM metagross as all time best steel attacker. Seriously, MM Metagross is the only pokemon in all of GO which will forever remain BIC for it's type and is relatively scarce if you are looking to trade for one. Yes C/P mewtwo as best Psychic attacker is technically unobtainable, but even the average joe currently has so many of those that it should be no problem at all to trade for them. I have 9 spare mewtwo right now (6 im saving for a friend, 3 I'm readily willing to give away) if you really want C/P mewtwo that bad.
TL;DR: Community day has ruined absolutely nothing so far other than your chance to obtain the best steel attacker/ gym generalist in the game. All spoken for... that's really not much that you could have missed out on.
Edit in response to "Why catch mons during the month?": Stardust. You need stardust to power up your CD/Raid mons that are actually good. Yes your "monthly grind" mons are mostly garbage. The game has always been that way. Grind for that stardust.