As @FinsToTheLeft notes, funding throughout the Gardiner’s history has been inconsistent, and maintenance costs money. But the additional challenge is Toronto’s climate, which ranges from icy cold winters to blazingly hot summers. Contraction and expansion are going to take their toll on an elevated roadway. Plus, in winter, salt gets dumped on the roadway (not just the Gardiner; all major Toronto roads get salted), which makes driving on the road safer at the time, but over decades, that salt also takes its toll on the road surface, the expansion joints, and the concrete that keeps the roadway up.
Under these circumstances, I’d suggest that the cost of maintaining the Gardiner, no matter who pays for it, is going to get to the point where it’s going to cost more and more. As I said, parts of it (at its eastern end) have already been torn down as too expensive to maintain given their use.