Hahaha love this thread.
My two-pennies:
21sts ARE one of the bigger birthdays for all my friends and family (UK). The big birthdays are 1-18, 21, 30, 40, 50... etc. Personally, my group of friends all had much bigger 21s (whether it be presents, going out, hiring a party space, weekend party) than 18s. At 21 everyone can drink, and hold their drink, and it remains a really special year for me with all my friends clubbing together through the year for a big, lasting present for each of us. I'm 39 and I still have mine.
It WAS mean for the bride to select this date. There's always a choice of venues, you weigh up the dates, the cost, the feasibility of everyone getting there etc etc. She had other choices she could have gone for. My brother is getting married on the weekend of my 40th next year. He was really worried he'd upset me- he is very dyslexic and useless with dates so didn't realise when he booked. I think he was right to be worried, but actually I don't remotely care because I don't plan on doing anything, my old uni friends (all the 21 club) are going to do a long weekend at some point in the year as a collective birthday event.
I'm banging on about the old friends because I guess part of why they're still big in my life is that year of 21's. It felt an important age to us, and so I really do side with the 21yo in their feelings.
I think actively leaving the bride's party on the night is pretty mean-spirited back. I personally think there will be fall out if it goes ahead in its current form so you should let your position and knowledge be known and then duck out of it. They should not leave, they should do their own big party the next weekend (in addition the family dinner or whatever). The girl could maybe wear a silly sash saying 'birthday girl' at the dancing at the end of the day if she really wants to make a thing of it on the day. By that point the bridal party will be drunk or exhausted and probably really wouldn't care- the main part of their day has all been a dream.
Someone mentioned anniversaries and birthdays clashing going forward- yes absolutely, this is a consideration too! What a silly thing this bride has done!