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I think the version numbers are only useful in determining whether your device is up to date, or if there's a bug in the new firmware you might want to revert to an earlier version. Aside from that, those number are irrelevant and certainly are not a basis for comparison between different series.
Garmin automotive units have gone through a slow evolution of features. Typically each new series has something slightly different. Looking back, the Nuv 600 series did not offer multi-point routing and did not record or display a track log. IIRC, those features were added with the 700 series. Then another series came along and added the speed limit display, lane assist and junction view came after that and so forth.
Around 2010 they completely abandoned traditional garmin multi-point routes and replaced them with "trips" that could only be created on the device itself. All the new models were unable to either send or receive routes from Mapsource or Basecamp. Why they did this is still a mystery. But there was such an outcry that they introduced one-way route transfer from the computer to the gps in the next Nuvi series. This was still very limited and the whole "trip planner" fiasco continued until 2013 (IIRC) when they finally brought back the full functionality of the old Nuvi models.
The 2012 models introduced an interesting feature of custom dashboards that actually allowed you to design your own map screen. Then in 2013, they removed that feature and never brought it back. http://forums.gpsreview.net/discussion/26442/making-customized-dashboards/p1
They did the same thing with custom speed limits, where the user could enter their own limit if it was missing from the map. Advanced pedestrian mode was something they introduced in 2010 (IIRC), it provided a different map screen and cursor and allowed routing via mass transit if you purchased a special map. A few years later they discontined those maps and removed the pedestrian mode from the new models.
In 2010 they introduced the Nuvi 3790 which was thin and looked like an iPhone. They made a big deal over this, it had its own website. And it was very impressive, I had one. True 3d view including three dimensional terrain and buldings. Completely new user interface. There was a new 3d map browser that used the multi-touch screen to tilt and rotate the map. They continued refining this flagship model with the 3490, 3590 and 3597 and then just dropped it. None of the current models have 3d terrain AFAIK.
For the appearance of the main map screen, the biggest change came in 2013 with a different "flatter" look (apparently Apple-inspired) and tabs that slide open. I think this has been retained on the new "Drive" series.
Now the changes to the newest devices all seem to be based on a bluetooth connection to your smartphone, things like finding parking places, etc. The newest hardware does look nice, with big bright screens. Have only played briefly with them in the store. The problem is, the dedicated GPS is dying a slow death and their sales are way down. They will need to stand out from smartphones somehow. I suppose an obvious way is making 7" screens which would be a bit too large for a phone. ;)
Personally, I'm very happy with the Garmin StreetPilot Onboard app on my iPhone 6s+ which has a 5.5" screen. The user interface is very similar to a 2012 Nuvi, with some features borrowed from other models.
Regarding your question about which models will be supported... roll the dice and that's about the best way to forecast what Garmin will do. See the examples I cited above, they were all offered on Garmin's flagship models and announced with much fanfare only to vanish later with no explanation. ;)
I'd go one step farther and say that the version number says absolutely nothing about functions and options. You can find out everything about those on Garmin's original product page for the device. Everything else is just a bug fix.Good to know that the version number of the firmware of an particular model doesn't say that much over functions and options.
In some cases functions could be taken away in the future, even though the firmware still supports them. One example would be mass transit routing because Garmin stopped making the special maps. Another example would be their "connected" devices like the Nuvi 16xx. Garmin stopped providing data for them, and then AT&T took down their whole legacy Edge network.
Another example would be MSN network features that used a special FM receiver. They discontinued that service later as well.
FWIW, here is the Nuvi 3790 product announcement from 2010 :) garmin.blogs.com/pr/2010/04/garmin-nüvi-3700-series-redefines-look-and-feel-of-personal-navigation-.html?activeBranchId=newsroom#.WZx0ha2ZPog
"the road ahead never looked so realistic, thanks to the unprecedented details of 3D terrain and 3D buildings. Whether you’re heading for rolling hills or urban canyons, the shaded topography and realistic landmarks provide unrivaled context and enhanced situational awareness."
Have to say, that makes me miss "the good old days" when Garmin was producing something to actually get excited about....