Yeah it's not like Britain rejoining tomorrow would make a sudden change in the UK's economic output, but it could at least stop it from continuing to deteriorate.
And while the UK GDP is surely on a downwards trend (at least comparatively) that ignores the fact that the UK is still the 6th largest economy in the world and is, i think, still larger than France. Obviously it'd be better for the EU to add a similarly sized country which is growing instead of slowing, but those don't exist. Turkey's GDP Is still 4x less than the UK.
Brexit has certainly helped EU cohesion, support for Finnexit or Deportugal has dropped significantly since 2016. However if by some fluke britain turns out to be a success in the next 10 years then it might reverse that trend. The EU can nip that in the bud by coming up with a Brentrance deal that isn't horribly punitive, because there are real policy and fiscal advantages to the EU in British membership.