To continue using the Harrier Gr1 as a case example.
4x SRAAMs, on a decent sub-sonic platform, in a era where most things didnt have flares (or even any flares I think) and few things were faster than you. When the meta was still quite prodominantly gun fighting. Then I fully understand why it went up in BR. Figting a Harrier Gr1 in something like a Javelin would be totally insane to me.
But since it’s addition. Flares are now common place, this mitigates the SRAAMs a lot, the meta has changed more to missile fighting, and the lack of CMs on the Harrier Gr1 which was a non-issue is now quite a major issue and now there are all-aspect IR missiles which destroy the Harrier in any aspect with ease within sight. (and yes SRAAMs were broken, but that was broken by a game-wide overhaul of missile physics more than anything else I think, it wasnt a SRAAM specific nerf)
Without changing a single thing about the Harrier Gr1, the Harrier Gr1s performance has changed significantly. So again. Short of literally never adding any new aircraft, or changing any aircraft +/- 1 BR of the Harrier Gr1, its performance will change. These days it probably should go down in BR 0.3.
BRs must be dyanmic and change as needed. We have also got new information for its loadout. Flame found multiple docs for the SRAAM in the national archives a year ago, that shine a new light on the SRAAM. They need a total ground up overhaul these days and its overdue by about a year. But when that change comes, my god, SRAAMs are going to be potent once again.
The only reasonable change they actually need to make is to make fixing issues, especially those that affect balance so much, a much much higher priority on premium vehicles. You also have aircraft like the J35XS which are in absolutely desperate need of a BR change, but the BR change is about 2 months overdue at this point. Again, that needs to be addressed ASAP.
A Tiral period or limited window of refund, would be good, allow players to see if spending £70 on a new vehicle is the right choice for them or not, but it needs major mitigations to make sure its not exploited
But no, adding an aircraft at say 9.0. realising it should actually be 10.0 but not moving it up because “people spent money on it” and watching it just clean house without effort is not fair or fun for anyone else. Quickly matches would just be that one vehicle, because that is all anyone could actually play.