Well no one has moved as far as you well know, but it's not unheard of in rugby for clubs to move base. Saracens have had about 3 "home" grounds in the last decade.
I'm not defending the move, but there are definitely less obstacles to moving a rugby club than there would be for a football club.
The reasons are purely commercial, if it were down to sentiment no club would ever move home.
So, what have we ascertained?
No Rugby club moves as far as Wasps.
Saracens have found themselves returning to London after failed attempts to leave.
(How many non London Rugby clubs have moved substantially?)
As for the more sociological elements, here are a few conclusions which are not unreasonable based on the above:
Even one move is not sufficient, a rugby club's tie to place is, supposedly, far less and thus the potential for them to move once more is greatly enhanced.
Apparently, the club's tie to place is insufficient for it to have a cultural impact on them moving.
A rugby club's role in increasing cultural awareness of place is insignificant.
Now, if all these are correct... why would a city council find it a good idea to sell to a rugby club, if it risks their football team also becoming nomadic? All this does is diminish the cultural value and cultural awareness of the city the council is supposed to be representing.
Therefore, it is a poor decision not just for the football club, but for the city. Cultural value will outshine financial value long term, after all.
(And as for the short term finances, we can't see if it's a poor decision or otherwise on base numbers until the FOI requests and appeals are made for the information to be released)