Going from 19-20 costs the same as 0-16. So instead of getting one new rank on your main, you could get a secondary weapon up to 3 ranks below your main. I'm not arguing that it doesn't take time. I'm arguing that because of the way the cost increases, you will at some point be able to massively upgrade a secondary weapon (Depending on how far behind it is), just for one single point on your main weapon. I was also talking about when your main weapon is "max" level. The 20 point trait increases your damage with 0.5% per rank (For DPS at least), and you can't relic it obviously.
Also, getting very high level with your artifact weapon is simply not grindable. You NEED artifact research to ever hope to get it maxed, and that is gated anyway. So, in the end, you can choose to keep a secondary weapon within 3-4 trait points of your main weapon at the cost of 1 point (30 main, 0 secondary vs 29 main 25 secondary, for example), and while artifact traits are powerful, they don't make or break your performance as much as you seem to think.