It's not completely accurate to say that price bridging is entirely a thing of the past, but it is true that bridging only happens now in a very limited form. By dating tickets, Disney has put a ceiling on the value of those tickets, so that bridging now only applies up to the value of that ceiling.
For tickets purchased from a reseller prior to the introduction of expiration dates in early 2017, tickets can be bridged up to their value in 2016. Newer tickets can be bridged to their value at the time of expiration.
So if I had a ticket bought from a reseller in 2010, that ticket should be valued at its 2016 price on an upgrade. A ticket bought now from a reseller at, say, $50 below Disney's advance purchase price price, will still be valued at the advance purchase price on an upgrade (ie. you will not lose the reseller discount).
Note that bridging does not (and never did) apply to tickets purchased directly from Disney (so if you have tickets bought from Disney in 2005 and you want to upgrade them you'll pay the full difference between what you paid for the ticket in 2005 and the cost of the upgraded ticket today) or to tickets discounted by Disney (ie. Canadian Resident tickets).
Source: Robo's "Ticket Sticky" on the strategies board.