And sure we're not quite there yet - the entire library of congress isn't online yet. But we all imagine its going to end up there, no contest.
The real issue is, does e-book loss (losing rights) compare unfavorably with real book loss (loaned out and forgotten; burned or discarded; degraded to unusability)?
Loaned books and not returned: same thing with files if cloud service will ever let you do that and you can't make a copy for yourself. However they could let us set an automatic return date. That will solve the problem. The point is: why bother if they won't make money out of it? Pay a little fee to borrow or to lend?
Accidentally burned vs accidentally deleted: same thing. Only the ability of backing them up is going to save you. Or a cloud service unless we are ruling it out for the rights issue.
Degradation: hard to say. Ebooks haven't been around for enough time to say how many of them I could lose in the next years. However I still have files from the mid 90s so if one really cares about them it's probably safe to say that they won't be lost. I can probably read some cassettes and floppy disks from the early and mid 80s if I bother to switch on their original hw (and if it works). However it's a vastly larger pain than reading an old book on aged paper. My parents have some of those books from the early 20th century and we can read them. Obviously they might have thrown away other books and it won't be the case if those books were bits.
Still: without cloud services are we going to be able to read epub or mobi in a 100 years? And with cloud services how many books won't be available anymore by then because of policies, politics, commercial rights, censorship?
In 2012 I bought some music in electronic format from Amazon. A bit later they introduced the Amazon music service. I looked in my purchased music section, and the music I'd bought in 2012 wasn't there, and if I lose the downloaded files it's not available to me unless I buy it again or the rights holders come to some agreement with Amazon.
It's possible that stuff will drop out of the cloud when the rights become impossible to work out. I'm not sure how likely that is with books, but it happens with films. The most well known is "Point Break", which now can't be shown in public by cinemas because no-one knows who owns the rights.