Well, this requires numbers. Like percentage. Do you have any or it is just an assumption?
Are they unreasonable assumptions in your eyes? May I ask why? They seem reasonable to me.
Anyhow, if we assume storage costs for YouTube are the same as what AWS charges per month (roughly 2 cents), this would mean that it costs YT about $2.4 billion per year just to store those 10 exabytes. I'm assuming that Google can cost its storage at quite a bit cheaper than 2 cents per GB at the immense scale it operates on, so let's halve that. This still means simple storage costs of $1.2 billion per year.
These of course don't include hardware replacement, other capital costs, or the bandwidth costs of all that video being uploaded (and downloaded).
Youtube's revenues for 2019 (so a bit out of date) were nearly 16 billion (2), so while they certainly have lots of costs beyond what I described above, they're also wonderfully profitable it seems.
1 https://www.productmanagementexercises.com/9018/estimate-tot...
2 https://abc.xyz/investor/static/pdf/2019Q4_alphabet_earnings...