I don't like the new sphinx design, but I've never liked D&D's winged lion 'sphinxes,' partly because sphinxes in pretty much every real-world culture they appear in have non-lion parts (usually human heads, sometimes even human bodies with lion heads IIRC) besides the wings, if they even have wings at all.
It's not a new problem - AFAIK all sphinxes being generic winged lions (which does not faithfully depict real-world sphinx iconography at all - there was an article saying they were changed to be closer to Hindu/Buddhist sphinxes at one point but either that article is incorrect or WotC needs to replace their cultural consultants, because Hindu/Buddhist sphinxes also have human heads) is something 4e gave us. I almost miss the weird eagle-headed hieracosphinxes or the goat-headed criosphinxes. Almost.
If I hadn't been told the art was for sphinxes and I didn't know what tressym were I would have just thought "wow that's a colorful winged lion, I wonder what the name for this D&D creature I don't recognize is?" Since I knew what tressym are my first thought was "wow, that's a pretty design for tressym" followed shortly by "holy crap that's a massive tressym."
Now my excitement for new tressym variants (you think I'm being sarcastic, I assure you I am not) is squashed and replaced with "yeah, D&D still doesn't know what a sphinx is - disregard the MM art." Which... hasn't really changed much at all, to be honest. I already felt that way with the 2014 MM sphinx art. (The 4e art looks like it's in pain just from sitting there.)