Did all the old people decide to retire simultaneously and then the world forgot about them instantly or something? Did noone ever seek out these supposedly famous seekers after retirement to learn from them? What about the slightly younger seekers who would have been newbies when the old people were active, did they not learn a single thing from them?
It's like ALL the old seekers are 65-year olds, and when they all decided to retire at the same time, all the 45-year olds had to start fresh with no knowledge. And then this repeated until the current age, so we have a bunch of young people with no knowledge about how to defeat the monsters. And all of the older people turned a blind eye to it as it was happening, so now the young people are struggling and probably dying because no one bothered to teach them how to fight. It'd be one thing if they knew about the tactics, but can't put it into practice is too difficult or if the old tactics became obsolete because there are newer and better methods, but it's neither of those.
This setting is as ridiculous as telling someone that one of the most famous Street Fighter streamers today has no idea what a hadouken is, because everyone who played it in the arcades in the 90s decided to never tell anyone about it, or that a famous Mario 64 speedrunner doesn't know how to skip Latiku on the bridge.
And how did the father grow up to that age without finding out that his parents are supposedly famous and strong people? Or is he their son-in-law, while the mother is their child? If so, how did they end up getting married and having a child without her ever telling him this very noteworthy detail about her parents?
Gramps retired 30 years ago. His son might not even have been born yet, and he didn't necessarily grow up within that culture. The Seekers back then were specialists, not streamers. The strongest among them might have been famous within the trade, but the general public wouldn't necessarily know about them.
Gramps' techniques are effective, but difficult to pull off and mostly tailored to his non-standard fighting style. So his old tactics
did become obsolete for mainstream use, because people at large no longer need to be that skilled. As dungeon exploring shifted from a matter of survival to resource gathering and entertainment, the safety organization likely enforced stricter rules to minimize risk. That would naturally lead to more standardized, tried-and-true methods with lower risk and a lower skill ceiling, built around party formation.
Mind that it was never stated that Gramps' methods are completely unknown to everyone. The manga has only shown that they're unknown to the public "viewers" and streaming dungeon delvers. That still leaves room for serious circles, veterans, or closed guilds where such techniques might survive, but guarded out of caution, regulation, or even self-interest.
Basically, it's not really collective amnesia. It’s limited knowledge transmission caused by generational shift, professional specialization, institutional standardization, and cultural drift.