The sales guy is lying and deceptive. Would be quite fun for then to try and install the equipment and then tell them to take it all out and leave after they can't deliver.
He made it sound like the highest speed tier was uncapped and you just get whatever speed you're capable of receiving. I should have asked him that, but I never think of these things at the proper time.
Full disclosure of the number of line feet the DSL would run can give you a decent estimate of what speeds you could see from such a so-called uncapped service and the quality of that copper is almost as important. Assume any copper older than 10 years might have excessive noise on it.
I don't know where you get your facts, but my home and the copper feeding my Uverse is over 30 years old. I get 30/5 profile on my service, and have yet to have any problems with my Uverse. I think your 10 year theory is nothing more than a random opinion. Let's stick to the facts.