I'm not so certain that users will be able to keep Aperture running without direct support from Apple unless they are basically 'freezing' their OS level and camera gear as it is.
There are many 'gotchas' in intimately tying Aperture to the OS and the hardware.
For example, if you want to run Apple Remote Desktop or any other software and stay current, oftentimes the updates will require a fairly recent point release of MacOS, not just within a few versions. If Aperture won't run on a later release for whatever reason, you'd be stuck on an earlier version of MacOS and Apple Remote Desktop. This would not be the best situation for security fixes and vulnerability issues.
Also, Aperture itself required later Mac OS point updates just update itself, and camera RAW updates are intimately tied to Aperture as well. Obviously there is something new coming that is breaking compatibility with Aperture, otherwise they would have let it keep withering on the vine like they have for the last 4 years. I'm guessing it is lens corrections, but what happens if you get a new camera and it isn't supported by your old version of Camera RAW, and the new Camera RAW isn't compatible with your old Aperture?
Also, we all know that many recent point updates to Aperture did *something* to our libraries during the updates. Old versions of Aperture won't open newer (updated) libraries. So what happens when you update Photos.app and it says "updating libraries" and Aperture will no longer open your libraries?
As an aside, there is a lot of confusion going on about what Photos brings, and what Apple is intending for pro photographers. Given that Apple had years to decide what was going on, this type of communication from them is appalling, since they are one of the largest, most profitable and successful companies on the planet.
If you look at apertureexpert.com, he has a post on the discontinuation of Aperture. Interestingly enough, he said an Apple PR Rep phoned him to break the news... If someone at Apple has enough sympathy to phone a third party Aperture site operator, you'd think that they'd be more prepared to discuss the migration and features of the new product, Photos, and properly mollify the Aperture user base.
Cancelling Aperture is one thing entirely, and I respect Apple's decision to do so. But the way Apple has rolled this out is really poor. Someone has really screwed this up at the mothership.