Again, you keep calling them tombstone weapons....but as of now, the only way to get the key item needed to get said weapon in the first place...IS TO DO FINAL COIL. Sure, coil might also drop better weapons.... but you keep making sound like SE is giving the weapon away for free...and that LOTS of people will have them. And yes, maybe the final chapter of the Crystal Tower will drop the key item as well...but that is months away.
So for now, for many people, (maybe not you personally), their Relic IS their best option. And as others have pointed out, even for you, it's likely that as a TANK, you won't see the key item needed for the weapon until at least every dps on your team has theirs. [I]Also, and this is important, you can beat the final turn of final coil with you Nexus. I get it may not be your 'BiS' at the time....but I could argue that between RNG having to work in your teams favor for the key item drops+ then needing to spend tomestones on a weapon instead of gear that is also a potential upgrade (BiS), the mild boost in stats might not be worth the invenstment. Because if you can beat the final boss of coil w/ the Nexus, and since that's where you're real BiS will drop, spending points on a weapon you likely use for a couple weeks while learning the final boss (if it takes your team that long), is likely not worth the time/loss of spending the points elsewhere. Especially since your whole issue now is that Nexus is replaced by it.
Oh and again, I don't know how to make this any clearer....just because a small percentage of the playerbase is challenging Final Coil while it's the newest and hardest thing, doesn't mean the majority of the playerbase is/will. For most players, their Relic weapon is going to almost always be their 'BiS'. Because most players will actually progress in the Relic questline faster than they progress in the coil questline.
PS: this still doesn't get to the meat of the matter though. You are misunderstanding what a relic is. A relic is an upgradable weapon. You will always have it by your side. All those other weapons in game? Good for a couple months and then you toss em. The relics MAIN saving grace is it's constantly evolving as you do. But the tradeoff is that at basically every step in the relic-progression, there will also be weapons of similar strength. Which is OKAY. It doesn't lessen the relic...because the point of the relic is to have again, a weapon that grows with you.