I've missed my chance to get the Gengarite last Halloween but I definitely do not have the same opinion as you. It most likely won't be the ONLY chance we can get the mega stone anyways, as can be seen from previous seasonal legendaries that we could catch, like Shaymin and Darkrai.
Mega Gengar is far from useless, I can agree on that. You can use Mega Gengar in Bosses, Sinnoh's Battle Tower, Suspicious Bot, etc, but I personally think that having the Gengarite stay as a seasonal quest is fine. It gives us, as players, a sense of achievement once we do get a hold of it, making it feel special.
If they made a new quest to get the stone, where anyone (or people who've completed a certain region) would be able to obtain the stone anytime outside of Halloween, then that'd just defeat the purpose of making it a reward for 2020's Halloween quest. Also, just a small detail, but Umbra, the Mega Gengar you battle in the Dark Realm and Battlefield, go hand in hand with the Gengarite, so it fits the theme.
I wouldn't go as far as say that only G3n's and Comrader's opinions matter. I think everyone is entitled to an opinion, each with equal importance. Both G3n and Comrader presented their opinions, and provided reasons as to why they think so, which is feedback as far as I know. This whole conversation is not useless, I can tell you that much.
If I'm getting this right, he meant time as in a seasonal event.
I can understand that some people missed out on getting the Gengarite, as did I, and that you want to spend as much time using Mega Gengar outside of ranked, hence why you made this suggestion in the first place, but in the end a suggestion is a suggestion. We're here to give you our feedback, take it as you will. If it doesn't get implemented, please don't get upset.
I missed out on catching Darkrai for 2 years, and managed just fine. I'm sure you can do the same if it doesn't get implemented.
Edit: -1