Yes, it's possible to win with less money than your opponent, but why would anyone want to take that risk?

The problem with money in politics is not that money guarantees a win, but that the presence of large donations distorts the entire incentive structure of campaigning and governing: Courting big donations means spending time with big donors (who expect access in exchange for their money) and when it comes time to govern, studies have shown that campaign contributions and lobbying are dramatically more influential to what gets proposed and passed than the preferences of the general public.

Focusing on the problems with presidential campaigns re: money in politics is missing the forest for the trees: All politicians have limited time to spend between campaigning and governing, and if they're constantly raising money the governing gets delegated to lobbyists.

(This is why people are always so shocked when politicians who don't accept corporate PAC contributions have drastically different priorities than those who do. Of course they do! They don't have to spend all their time hanging out with corporate lobbyists!)

This doesn't really speak to Citizens United though. The nature of Dark Money is that no one knows where it comes from, so politicians cozying up to their donors is not actually the particular concern here.

(Also, there has been the opposite trend, which is that more money than ever comes from private donations from billionaires and other wealth.)