It's not directly related to the torrent or its content no. It's more related to the potential bugs in Transmission that might be exploited to propagate viruses.
Since Transmission has to exchange data with un-trusted parties, before knowing whether the data is relevant to the torrent you are downloading, anyone could exploit bugs that exist in the parsing of these messages.
So running Transmission as a dedicated user limits what an attacker may have access to once they take control of Transmission through the exploit of known or unknown bugs.
Obviously, this user need to have many restriction in place as to prevent the attacker from installing malware permanently on the machine. And when you copy over data that has been downloaded by Transmission, you'd have to make sure it has not been tampered with by the attacker in an attempt to get access to the data available to your real account.
If you just use transmission occasionally, not on a server, I would not bother with it. Either use the flatpak version for some sandboxing and similar security guarantees as having a dedicated user running Transmission, or use an up to date version (the one from your distro should be fine) and don't leave it running when you do not need to.
I can of agree the focus to make Linux easy to use is not exactly on the right things. There is a bit too much of a "make a GUI of everything". Which is not wrong per say, but should not be the goal. More a mean to an end.
I disagree that users won't do stuff on their own. They will, but they will allocate very little time to it, on average, especially when compared to a tech savy person. And that's just because their computer is a tool.And if they cannot make their tool work for what they want to do, they'll find another way. Or deem it impossible.
I think distro must make mundane tasks such as system maintenance hands off. As an opt-in option not to upset power user. But things such as updates, full system update, disk space reclaiming, ... should have a single "do the right thing without being asked to" toggle. Things a bit more complicated such as printing/scanning document should be more context aware. A bit like on smartphone where, if you have a document open, you can select print and, if no printer is configured, you have the option to add one there and then.
Immutable distro have made good progress on that front IMO. But we still need better integration between applications and the Desktop Environment for things like printing, sharing and so on. I'm hopeful though. Generally speaking, things are moving in that direction. Even if we can argue flapak and snap are a step backward with regards to the integration of the DE, this is also an opportunity to formalize some form of protocol with the DE.