Welcome to game development, where existing models are often redressed or retextured to save time, money and client-side processing, the later especially crucial for an online game with many hundreds or even potentially thousands of models on screen at once - having every model unique slows things down as the computer has to 'draw' every one individually, where as reusing a common model means it just has to copy the model the required number of times and simply change the texture as required, saving a lot of processing time and power.
Also the blasphemy models are actually more or less the same ones we saw earlier in ShB (aka Amaurot) which makes perfect sense lore wise. They do add unique models, but even from the beginning a number of enemies were actually designs from other FF games as fanservice, so that's another reason for 'recycled' models (the trash mobs in the second world visited in the Dead Ends for instance, are actually retextured enemies from FFXIII).
As for gear appearence recycling, again, this is normal, the same basic model will often be used by multiple pieces of gear with differeing textures (in FFXI this was particularly common), and has been the way since... 1.0, so again there is the way things are actually done. Yes it's nice to see new designs, but they're always going to reuse similiar designs for gear with new names and textures as time goes on out of pragmatism.