I generally like questions that start with "what can possibly go wrong", but to be honest, this one isn't super-scary to me. If this is the reason you have backups at all, I'd consider this a huge win.
The risk I see is that the attacker that disables your anti-malware solution potentially impacts your other defense against malware (e.g. crypto-ransomware). Practically speaking though, you have backups, so ... could have been much worse.
The caveat is that you do still need to manage your backups and protect your backup media, but that's true regardless of your backup tool.