The fact that Gran Turismo 6 wasn't just ported to PS4 is mind blowing. Gran Turismo 6 seems like the Command and Conquer 4 of the racing world, not in quality, but in that no one remembers it came out or exists.
Gran Turismo 5 was terrible yet somehow it sold a ridiculous amount of copies (10m+ I think). I wonder if Polyphony burned through their goodwill with GT5 and that is why GT6 came and went.
The brand is still fine (A PS4 GT6 around launch would probably have had way more legs) but Polyphony has completely lost the plot since the beginning of HD consoles. They've been lagging 3 years behind what would seem to be the best timetable, and even then can't keep up with their own promises or with the competition in quite some departments. I wouldn't bet on it but the mismanagement could catch on to them with Sport, considering there's now some competition on their own platform. No bother though : you know middling sales would already be handwaved on the fact it is a spinoff, just wait for the mainline title (in 2019).
Late reply and I'm not sure if I did reply to this earlier but: It already has. just look at GT6's sales compared to 5's. You could say "but that was at the end of PS3's life!" and
sure, but it was also
two years after GT5? The same GT5 that took... what...
four years to release? The same GT5 that had an HD demo that they pulled, put out a prologue like 2-3 years before the
final release happened? The same Prologue that they didn't bother to use patches to update so people could at least see some progress from them on it?

There's Project Cars now. There's Forza (even going PC!), which has had more features than GT for ages now. There's Assesto Corsa or however it's spelled on PC (and coming to consoles?). Polyphony hasn't been relevant in
two generations. Just look at the sales chart: GT3 A-spec which was a launch or near launch title for PS2 was the highest selling for the series.
GT5 was just the last straw with me (and I'm sure others) in that they hyped up many things (Go-karts! NASCAR! Top Gear™ Test Track!) and under-delivered (course creator is badddddddddddddddddd and extremely limited) and didn't even match features with Forza, it's direct rival because Kazunori thinks livery's on cars is tacky and everyone should have the same Salaryman Civic or whatever Nissan shitbox look.
Honestly, GT6 should've been GT5's content with faster menus and a better engine. It's crazy that a lot of GT5's content (IIRC the DLC and some tracks didn't port over) wasn't used.