There are numerous ways that trees on Ancestry can be preserved; downloaded GEDCOM's can be uploaded to other sites, including familysearch.org, or any software product of your choice (RootsMagic for example). Another way, for Family Tree Maker users, is to 'sync' your trees to the software, all images for the records your tree contains are downloaded too for off-line use at any time; the images can be 'copied' to your computer from the software as well. The fact you can have copies of such things as parish records, immigration records, census records and so on, transferred to your computer through syncing your Ancestry tree to the Family Tree Builder software is avery good reason to buy the software now even though support for it ends in Dec 2016, then leaving Ancestry is made easier.
As for Ancestry, my view is that they tinker with their software continuously causing bugs and glitches which give it's users recurring nightmares at times - there have been some monumental errors caused by their tinkering in my time with them (about 15 years). Their time would be far better spent correcting all the errors in transcription, a far higher proportion than sites like findmypast; getting the location of thousands of individual parish records (Rochdale instead of Royton - it took them well over a year to respond to notification of the error), and entering Christening dates as birthday dates are two examples.
The biggest problem with Ancestry though is, they have the best tree building software available! I tried to leave them but just wasn't satisfied with any other site. So, I have a subscription to findmypast (world) to use when Ancestry lets me down, which is all too often! I have synced all 58 of my Ancestry tree, with copies of all the record images, and downloaded them as GEDCOM's; soon I should be able to leave Ancestry behind. I intend to have my main trees here on WikiTree, but transferring them and adding all the details and sources is almost a lifetimes work. My other trees are for my Guild of One Name studies projects.
I've found that the best source data is from familysearch.org, the other sites just pay lip service to the concept.