Digital Ocean allows only one account to be in control of a given domain. If you own a domain, creating a record keeps anyone else from trying to create records for your domain. I doubt that hijacking a domain by creating records in Digital Ocean and then re-directing the registrar to use DO’s name servers is a thing, but if it were, this would help.
Right, but if you’re not going to change the address for years, there’s no advantage of having a TTL of 60 seconds. And it makes things slower for the user every time the TTL expires. Of course, if you have an unplanned need for a migration then, yeah a shorter TTL is what you want. And only if you have a short TTL does the speed of Digital Ocean or CloudFlare make any difference. For the bulk of my customers, if there is need for an unplanned migration, the TTL of their domain is very likely the very least of their worries.