That wasn't The Cardassians and the Romulans, it was the Obsidian Order and the Tal Shiar. Big difference. Also, it wasn't an invasion, it was merely a strategic attack to decapitate the Dominion. There was no intention to expand, at least not from that action. It might have brought some much-needed internal prestige, though...and given the OO a lot more clout in things.
Eh, gonna have to agree to disagree a bit. It's true that the Tal Shiar and Obsidian Order were not the regular military forces of the Romulans and Cardassians and were intent on wiping out the Changeling leadership, but had the expedition been successful, that would have been justification for each power to start colonizing the Gamma Quadrant. That is what empires do after all.
But regardless if you agree or not, that event was yet another blow to them as a power, as it set in motion a series of events that lead up to the Klingon-Cardassian war, which finished them as a power player. That was the main motivation for Dukat making the pact with the Dominion: as he put it, the alliance was intended to "make Cardassia strong again." That definitely shows that the Cardassians had at one time been a power to reckon with, but after having fought a bloody war with the UFP to a stalemate, a failed expedition against the Founders, and finally the Klingons, they became a second rate power.