I think the fact that the rescue and subsequent mad scramble to blow up Starkiller Base consisted of only one type of fighter isn't too big of a stretch. Squadrons of fighters tend to be made out of the same type just due to logistics and efficiency as a unit. If that's all they had, that's all they're gonna use.
In the end, the story threw in what it needed to as a backdrop for Poe's ability, and it satisfied fan's thirst for more XvT. There's gonna be a lot of room to bring back everything else and/or see the upgrades in tech in the intervening 30 years.
Again, going back to the parallels to the Legends/old EU timeline, X-Wings WERE still in service at the time of Dark Nest trilogy, albeit newer, better models.
The X-Wings in TFA are significantly different in ways - under-dock/unassisted loading of Astromech (lol Luke @ Dagobah), the anti-personnel underslung cannon, the whole S-Foil design being different (notice in the picture, the lower foil is in front of the upper one, so they close in front of each other rather than flat against - the engine design is also different and closes to form one cylinder in the front of the S-Foil) to probably a generation's worth of upgrades across the board that aren't immediately apparent.
On the other side of the coin, TIEs used to be disposable one-man fighters for disposable pilots. You needed the helmet because the cockpit had no atmosphere. There were no eject seats You couldn't enter hyperspace with them (presumably you can in TFA since they were planning to escape in one).
Perhaps advancements in technology made TIE designs modular enough that it was possible to have multiple configurations in the same basic hexagonal fuselage type made the bomber and intercepter variants redundant. (Hey, they stole a TIE with a gunner seat and backwards-facing lasers). Perhaps not and we just haven't seen the bombers and interceptors and whatever constitutes for Advanced and Defender classifications.
It does go back to the whole 'bring in some recognizable things from ANH but keep it simple' concept - which I don't particularly agree with, but which works - and likely saving cooler stuff for later.