I'd like to find an easy solution to that problem. I've often heard what you just said. I've had students who came out of immersion and told me the same thing, which is that they didn't have the opportunity to speak French after high school.
This is obviously a personal choice. Now it's possible to watch French-language television in all regions of the country. French radio stations are also available across Canada, and you can establish French conversation clubs as well.
I speak a little Spanish. My Spanish becomes much weaker when I don't have a chance to travel to Spanish-speaking countries. If I wanted to maintain it, I would read and listen to things in Spanish. It's a personal choice. If people want to maintain their knowledge of the language, it's up to them to use the means at their disposal.
In Canada, several aids can be used to do that, such as books, newspapers and French language radio and television stations. I regularly watch TV and read in English. That's what helps me maintain my knowledge of English, but it's a personal matter.