For mage, shared streak is not at all useless, yes it overrides selfish streak, but selfish streak also overrides shared streak. Each proc every 3 consecutive spells, shared gives a 4% damage boost to everyone and selfish gives an 8% damage boost to yourself.
Assuming 2 people with about the same casting speed and some variability throughout the fight, one running selfish and one shared, the “selfish” mage will on average have the shared streak boost half the time and the selfish streak boost the other half resulting in a 6% damage increase overall, which is less than the normal 8%, however then everyone else in the party also get a 4% boost, which more than weighs up for the 2% loss of the mage with selfish streak.
If you have a lot of mages in the party, then it might not be worth it anymore, but for most of the party compositions I run these days(in raids at least), shared streak is a net positive.
That aside, I do feel like in this case, shared streak should not override selfish streak, however, selfish streak needs to be able to override shared streak. aka, the better buff should take priority.
The same logic could also be applied with shields, or any other buff/debuff that has variability in it’s effectiveness, better buff taking priority, if a weaker one is attempted to be applied.
But it’s still not a perfect solution, and I don’t know if it’s better than buffs overriding each other as they do now. A fresh, but weaker shield, is always better than a stronger shield that will run out before the next hit. If an enemy is frosted but you need it to move faster, you can currently cast a frost 1 to override the frost 2/3 slow and as a result speed up the enemy. This wouldn’t be possible if the better buff took priority.
Likewise, sometimes you want just a frost 3, so you should to everyone not to cast frost 2 cuz it’ll override it, in that case a system where the better buff takes priority would be better.
Point is, what you’re suggesting would work just fine and be helpful in several situations, but what we have currently also works fine, tho it is a bit less helpful in many situations, comes down to community adapting to new rules. However, I think the current way it works is somewhat more “open”, it doesn’t exactly help you, but you can work with it more freely I feel.
(except selfish/shared streak, there you don’t really have control other than “don’t use it lol”)