clearstream
(He, Him)
I share your feeling on this. Why have a limit unless it offers interesting game play? And you don't have to throw out the whole rule: you might decide to retain the uniqueness constraint (a player can benefit from only one copy of an item listed in the DMG as requiring attunement). And scaling feels like it will end up either not relevant or about the same as a fixed limit unless you also scale the items themselves into some kind of tiered system.And so I reiterate: if you plan on increasing the limit to six, consider simply removing it instead.
I dislike suggestions of linking attunement to character stats as that seems to be offering players with great stats a win-more benefit. Or if they are choosing stats then whatever stat links to attunement gets extra weight which punished players who choose classes that need to prioritise other stats: it will be kind of like saying that some classes naturally have more access to items than others. That could be okay in the right campaign, but overall I dislike it.
Overall I would suggest something very simple but still limiting e.g. scale 2/3/4 against campaign magic level. Or something more complex like tiering. Or dropping altogether... possibly retaining uniqueness.