It all depends on the definition "isolated".. but I tend to disagree here. It could mean "everything". >99% of the audience on Internet aren't network experts and aren't security experts either (I won't say I am one).
There are probably several commonly used libraries used in Dahua's firmware which are pretty vulnerable after two years. So when there are no ports forwarded (IPv4 NAT: either manually or via UPNP) from the Internet on your home router to your unpatched camera (could be a definition of "isolated"). Then someone (your dad/friend/etc) enters on the same network at your home, where your Dahua camera resides, with an (knowingly/not knowingly) malware infected device: it still can infect your unpatched Dahua camera locally. Off course this is only one scenario, you can think of several probably.
The chain of security is as strong as the weakest link. So I prefer and would advise to keep things up to date when possible, from a security perspective. But hey, who am I?! Everyone has to make his own choices off course..
So would you update the Dahua 49225 PTZ and accept losing the ability for it to autotrack as Dahua deliberately removed it with firmware updates? Hikvision did the same thing with their DS-2DEA425IW-DW PTZ. Hikvision removed the the ability for a plate reading camera to read USA plates, which is a big deal if you are in the USA.
I get what you are saying, but most here have their cameras on a completely separate IP address subnet thru either a VLAN or dual NIC setup. Someone can come in and connect to my internet and cannot access the cameras as they are not on the same network as my internet. No IP scanner tool will find the cameras. Further, nobody is connecting hard-wired into my system. If they connect, it is thru the guest wifi that is isolated.
These same people also do not have Alexa and other IoTs in their house.
When I say isolated, I mean thru either VLAN or dual NIC. Some will consider their cameras isolated from the internet if they have blocked their IP and MAC address in the router or implemented the parental controls feature in the router, but the cameras all have the same IP address subnet as everything else. Maybe in that scenario your hypothetical is correct.
But if there are no update notes on what is being provided in the update, it is wise to wait and see what others experience before taking that jump. We literally have dozens, if not hundreds, of threads started when people updated their 49225 PTZ and lost autotracking.