That is a very tough answer to get an accurate answer. As far as my personal experience, combustion engines do lose efficiency as time goes. A lot of it has to do with maintenance by the owner, though. Components such as plugs/plug wires, air filters getting clogged, carbon buildup in the chamber and intake, etc etc. Then you have components not in standard maintenance such as clogged injectors, A/F meter, oxygen sensors, catalytic converter clogged, and any amount of other sensors failing (engine checklight) that owners don't care to fix.
A huge chunk of energy loss related to operating an automobile, is in poor tire maintenance..A car with old and/or under-inflated tires, will render very poor mileage as compared to what the sticker said when it was new. Obviously those factors would, (or should), have a direct effect on an EV's range as well.
Those things said, an EV has many less moving parts (?), and save for the battery, there are, (AFAI can guess), simply less things to break.
This is all sort of an academic discussion, which we, (myself included), should have confined to the batteries in a EV alone. Moving on anyway.....
But, so many other factors enter into play when considering keeping a vehicle, long term. Perhaps even going so far as to considering the longevity of the clear coat, and quality and durability, of the weather stripping.
My point being, it's really two completely different matters, whether exchanging vehicles based on mileage, or on time retained.
The Teslas on the road thus far, have been, to a large extent, toys of the well to do. But when they, and other EVs hit the mainstream and used markets, lots of other factors besides battery life will come into play.
Can you still get parts for it?
"I don't have a garage, or even parking in front of my row house, how do I charge it"?
I could go on about vast changes to infrastructure which would be required for mass adoption of EVs, but I won't. After all, it's all about the batteries.
