I wrote "It has exactly the same upside / downside" and yet almost immediately I get a reply in which the author with no self-reflection has imagined there are additional downsides to STARTTLS.
As others have explained an on-path adversary who'd be able to rewrite packets in order to stop STARTTLS can even more trivially block port 465 altogether. In fact cheap appliances can do the latter but can't do the former, so if anything STARTTLS very slightly improves this.
If you are imagining "Well I could have a policy of only using port 465" well, sure, but you could just as easily have a policy of requiring STARTTLS. In both cases an adversary can deny you, and your only options are to re-think your policy or give up and not deliver email.
As others have explained an on-path adversary who'd be able to rewrite packets in order to stop STARTTLS can even more trivially block port 465 altogether. In fact cheap appliances can do the latter but can't do the former, so if anything STARTTLS very slightly improves this.
If you are imagining "Well I could have a policy of only using port 465" well, sure, but you could just as easily have a policy of requiring STARTTLS. In both cases an adversary can deny you, and your only options are to re-think your policy or give up and not deliver email.