Your electrician/consultant seems to be raising two concerns, one is the age of the receptacle, second is whether it was ever a good idea to install a 50 amp circuit on the subpanel.
The first concern is not a huge deal as it can be solved by simply replacing the 10-50 receptacle, which essentially is DIY although your city may require a licensed electrician.
Second concern is a larger one, they are basically saying that installing a 50 amp circuit on the subpanel was a mistake. I don't think I would dismiss their concern out of hand but I don't think I would just take their word for it either. You need to get at least two more opinions on that before shelling out that kind of money.
Even if the concern turns out to be valid, your solution of using a lower amperage EVSE might be all that is needed. 16 amp is plenty for Clarity. Any electrician that you talk to that feels that a 50 amp circuit is too much for the sub panel, mention that you will only be using 16 amps and see what they say. They may say it's still not a good idea because leaving the 10-50 outlet could mean that later someone will plug in a 40 amp EVSE. Tell them you understand that but as long as you only run 16 amp is that safe, and if they say yes then you are good to go.
Another option if they really insist that you shouldn't leave it with 10-50 would be to downgrade the circuit by replacing the 50 amp circuit breakers with 20 amp and replacing the receptacle with 6-20. That would mean using only three of the four wires in the existing cable, unless they are against that also, but that option would be much cheaper than running a whole new 10-50 circuit off the main panel. Of course future proofing is another consideration, if downgrading to 6-20 costs almost as much as installing a new 10-50 then for future proofing the 10-50 might be worth the extra investment.
But hopefully you can get second opinions that tell you it is safe to run 16 amp on the existing 10-50 outlet, then you don't have to worry about it until later when you buy a Tesla!
Last edited by 2002; 06-29-2019 at 10:52 AM.