Personally I don't think 5 is a bad choice for relics. Sure, some linkshells may need to work a year on it to get it for every one of their relics, but if they have people with four relics in it, I don't think that's too much to ask either. If someone can farm Dynamis every day for months on end to obtain a relic weapon in the first place, then repeat this again for three more relics, this hardly seems like an unsurmountable challenge, neither in effort nor duration. Only the focus will shift from solo efforts to linkshell efforts.
And don't even start with "nothing but Dynamis for a year", this is a 2h event. Even the most casual of players will be able to find a LS they can join regularly for this. These are, after all, the best three weapon classes in the game we're talking about, and considering that, I don't think a year's casual effort is too much to ask to get an entire linkshell upgraded on several weapons per player. Not to mention that this is still a gross exaggeration for the vast majority of linkshells in the game and most will likely be completely finished after a few months. As such I really can't find a reason to complain about 5 items per upgrade.