They canned the announced DLC because of creative differences between Matsuda and Tabata which left to the latter leaving the company, and because DLC ~3 years after launch is basically never a good idea for any game. The fact that the tail end of the second wave of DLC was deemed not viable due to a multitude of factors, doesn't mean that XV, and even it's first year+ of support was not a success, especially when Squeenix said they broke even day one/day zero and it was part of the reason they had one of their best financial periods ever around the release of the game.
In terms of sale prices, this is basically every blockbuster game ever. Give it a couple months and you will usually be able to find a game at half off if not more depending on where you look. We see the same thing in the Inqusiiton thread where you have users saying "But how many of those 12m copies sold at sale prices huh?". The answer for any game which has sold 10m copies is usually "a lot of them". At a bare minimum they sold probably close to 5-6 million copies at full price, which is an order of magnitude more than what a lot of games do.
FFXV, by basically every metric, was a runaway success for Squeenix.
Importantly XV had massive legs. This isn't a case of like Resi6 where the game sold a lot up front (and even then sold less than capcom forecast at launch) and then never sold much more. XV sold like 5m at launch, 1m+ within another month or so when all reviews and whatnot were out, and then like a 1m and change a year for next several years. This isn't including DLC sales either.
To believe that it was all hype or something you'd have to believe that somehow the game continued to sell in spite of presumably negative word of mouth. To believe that it meaningfully damaged the brand you'd expect to see a drop off for the next title. Unfortunately the game was/is Squeenix's best PC launch, with very positive steam reviews, and 7 Remake sold perfectly fine for a single platform with legs comparable to XV.
The constant relitigation of "was 10m copies actually a bad thing" is always so absurd. You'd think that people would be able to say "huh game sold well even if I don't like it". I literally have to do that with basically every Pokemon game lmao. It would be like if I went through Metacritic reviews for XVI and started arbitrarily excluding them for reasons to say that actually game didn't review well lol.