I own perhaps a dozen TB1/2 peripherals, USB2/3 in A, B, micro form factors probably in hundreds. In outbound working environments I probably need to interact with USB-A on an hourly basis if not more frequently. This alone justifies the 2015 as the more ideal solution in foreseeable future. Having only 2 USB3 ports on it is indeed semi-limiting, but in mobile situations where I need 3 ports or more is rare, in which case a simple USB3 hub can be acquired everywhere even if I forgot to grab it. And in local setup the MBP is docked to powered thunderbolt / USB hubs anyway.
If I had a 2016/17MBP in place, I would need 2 USB-A to C adaptors at all times, probably an HDMI / DP multi out, and a multi card reader. If they are left in the bag they are subjected to be forgotten / lost / taken by coworkers. If they are permanently plugged to the MBP then I may as well have never gone USB-C.
By the time I need an upgrade to the MBP, the rest of the tech world will have much larger USB-C presence, and due to supply and demand all such peripherals, cables, adaptors will be cheaper as well, and with more reliable and proven options. If a new Mac could offer the same kind of internal performance bumps like they used to, then you may have a case where to "embrace the dongle and enjoy a new Mac", but for my use-case that's simply counter-productive. By the way my main mobile machine is the MBP2015 but I still keep a 2011 as a Swiss-knife / trouble-shooting machine, since it has TB, FW, ethernet, optical drive, USB2(always hope it's 3).