[Bluebottle]

Some cheap bikes have no filter at all.

By 10k miles a Mk1 PCX 125 I looked at had lost 20mph off its top speed due to belt/sheave wear, seemed standard for the type.
Schedule belt replacement was 12000 or 12500 but it couldn’t keep up with traffic by 10k.

 

[Bluebottle]

I think you’ve assumed a conclusion that I haven’t made anywhere in my post.

 

[Bluebottle]

Good point.
Piaggio X9 scheduled belt changes 6000 miles (ouch).
Look at their ventilation.

OOF!

A lot of the small Chinese bikes use the ubiquitous GY6 unit. Belt change is 8000 miles on those (usually with a plastic debris strainer directly on the case but some have a duct into the body cavity, it varies).

 

[Bluebottle]

that would be heating up pretty good

If you are concerned about heat:
One of the best things you can do is use a top quality belt like the OEM.

Aftermarket belts can generate surface temps 50 deg (90 F) or more hotter than high end/OEM ones at the same load.

 

[Bluebottle]

OEM CVT is pretty sturdy and doesn’t give much trouble.
Belts don’t often fail early unless they are cheap knock-offs. However, like tyres, they don’t like sitting around doing nothing.

Visual check is useful but not a guarantee, it does add to confidence that it’s OK.

Belts can go bang, which is messy and can bend things sometimes. Plus if it goes while you’re exiting a bend/overtaking or it locks your wheel up it’s going to hurt. That sounds scarier than it’s intended to be (I suspect other, more compact units are more prone to locking up)
If it’s a named belt, in spec, flexible and visibly good - I wouldn’t think twice about taking it out.