That's exactly my main gripe with online accounting software...and none of the players are "too big to fail". Any could be sold or go belly up, despite trying to appear like they'll be here forever. So I've wrestled with this long and hard before making the switch. (Maybe I gave in to "the dark side of the Force"?)

But except for old desktop editions the same is true of QuickBooks:  you have to pay their hefty annual fee to keep using it unless you can continue using an old version that's no longer being updated. I've looked for years for a good desktop replacement for QuickBooks and found none that suited me. 

You have two big investments in accounting software:  one is the data you've accumulated over several years, and the other is your investment in learning to use it--not just use it but to be proficient at using it. And you have to give up both if a software product "goes away" for any reason. I subscribed to ZarMoney for a full year before switching to it, and what I found is that the things it does are structured a lot like QuickBooks desktop. (I think it works more like QuickBooks desktop than QuickBooks Online does.) So my years of investment in knowing how to do things with QuickBooks desktop are significantly transferrable, once I found where everything was in the menus.

I looked at Xero, and FreshBooks, and several others, and all had capability shortcomings (or high-cost shortcomings!) for the things I want. For instance, with many products you have to pay a lot extra per month for a decent inventory system, which is included in the base monthly fee of ZarMoney. (I use the inventory system for grain and resale livestock inventories, as well as to keep a fixed asset list/inventory.)

I don't plan to ever buy another Intuit product because they treat their customers like cows to be milked, not respected, and charge a lot for the privilege. Xero must be capable because a lot of people use it. But I didn't like how it is organized (very different from QuickBooks) and it has some odd limitations--like their oddball equivalent to QuickBooks' Classes. And it seems too "busy", as if a lot of the features were tacked onto the basic design rather than being well integrated with it...I just didn't like it, or its price. FreshBooks was simple, but short on features. And if you search its history online, there are a lot of customer complaints about serious accounting-level bugs (not just little gripes) that have taken forever to get fixed--leaving customers dead in the water. Zoho was pricey once you added all the modules/features I wanted.  ....and I've forgotten how many other products I looked at.

So back to your point...the bottom line is that I had to change my thinking about long-term access to my accounting data. (1) My plan is to have a complete (printed and Excel files) backup of my data every 6 months. (Yes, more often would be better.) So if ZarMoney ever goes away I at least won't lose basic tax records for the part of the year I've backed up. (2) How many of us really need to look back at records that are more than a year old? Sure, I appreciate being able to search for some old transaction that happened 5 years ago to see what I paid for something, etc., but a lot of that is mostly about curiosity. I can do without it if my subscription to anybody's software abruptly ends.

Finally, ZarMoney is not perfect. It has a few areas where I wonder "Why haven't they cleaned up/fixed this?" But I said that about QuickBooks too. So I just have to let those things go, so long as they are not a bug that affects my accounting.