Fastmail's domain feature is the killer app for me. I have my @firstandlast.com secured for $7/year or something, and I can make anything@firstandlast.com and if someone starts spamming me I can just block it.
Spammers are going to find your domain name and spam it at obvious usernames anyway (eg contact@).
Besides what use-case do people have for throwaway addresses? In my experience in most cases the addresses you use aren't throwaway at all.
Fastmail does subdomain aliasing and I've been using that for years with my own domain without issues. Every subscription I have has its own email address. I don't need someone else's domains for that.
I don't think it's a given that spammers will find your domain, if you only provide your email to real people, and give generated emails to online services.
Email addresses that I use now look like this: reddit@subdomain.domain.com
If this leaks, I can track the source and I can bounce messages for this address.
While what you're saying is possible, a spammer needs to target you personally and that's not cost effective. It's not easy for them to try every possible English name at that address, because then they quickly get blacklisted.
Spammers collect addresses via scripts that crawl the web or via data leaks. It's more cost effective for them to get addresses that have been validated. All the spam I get are on these aliased addresses, biggest problem being the one I publish on my website, which I change periodically.
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I like using my own domain even for aliased addresses because I can change service providers on a whim. I love Fastmail, but if they ever piss me off, I can change to Google Suite or whatever over night, the only thing required is some flexibility in setting up aliases.