As a current franchisee fairly new to the company, I can provide a little background here.
First and foremost, to answer the OP definitively, yes, as far as corporate is concerned, $5 Footlongs are gone forever.
Subway corporate is actually kind of lost right now. Sales have been declining for the last 3 years and they actually have no idea why and are floundering blindly trying to fix it.
Concerning the decline in sales, I believe there are a few reasons they are consistently dropping.
They got caught off guard by the fast casual movement (which interestingly enough pretty much hijacked subways business model). There isn't any single new restaurant chain that can compete with Subway's presence, but the aggregate is eating up market share.
They implemented a whole bunch of cost savings measures to compensate for years of $5 Footlongs (lower quality meats for example), in order to try and improve franchisee profitability due to rising costs. This lowered the quality of sandwich when compared to the up and coming competition.
The company and franchisees made so much money off the $5 Footlongs through about 2012 that corporate thought they could expand indefinitely, thus over saturating the market with stores, many of which are run by sub optimal franchisees. (absentee owners for instance)
The economy has improved. Subway did so well before because the economy was in the whole and they offered a healthier seeming option at a low pricepoint.
So now they have so many stores that very few have the volume to survive a real rebranding where they significantly improve the sandwich quality and service. Many of these stores are run poorly because you don't have the profit margin to hire quality managers and most owners have to own multiple stores to make a living off them and finally, the food is getting more expensive with no significant change in quality.
I actually have no idea how Subway can fix all this but I'd love to hear reddit's thoughts on the matter. I have committed a good chunk of my life savings to this business and would love to see them turn things around. I just don't have high hopes at the moment.