Will they eventually force us to use yosemite?
No, later this year there is likely to be a new version of OS X and then they will try to
force persuade you to use that one instead on Yosemite.
Regarding the original question about Mavericks, I will attempt to give my best guess to add to all the others.
Since every new Mac comes with internet recovery, I expect they would need to support the release installed by internet recovery, until those machines are no longer supported under AppleCare.
The AppleCare support coverage for the Mac is three years
from the date you purchased your hardware (at least in the US and probably most other countries). That means, if there are still new mac retail sales with mavericks installed (and I'm hearing this is still the case), then I expect that there will at least be security updates for mavericks until the beginning of 2018.
Just remember, "support" doesn't mean new features.
By the way, I would recommend that everyone should download Yosemite before the next release, so that Yosemite will appear in their App Store purchases. Then they will have the option to download and install it later. Who knows, perhaps Yosemite will be working a lot better by the time they get to 10.10.3 or 10.10.4 and the next release will be worse?