I can see where the resistance against Spell Versatility, esspecialy for a sorceror, comes from. It does definatly step on he toes of the Wizard both mechaniclly and lorewise. But both of those factors get pretty overblown by many people, I think.
As far as in-game explanations go I don't think it's a far stretch to say that a Bard can dig into some old texts or rehearse some new songs which would translate into a different spell. A Sorceror's magic has, traditionally speaking, always been definited by the fact that it's inherent but the class is nothing if not mutable. If a sorceror spends a night bending the weave to his will (or his body to the weave), perhaps preventing the regaining of hit die that night, to change one spell into another then I can go "yeah, I can see how that could work". It's not like tech in our own world doesn't evolve.
It also doesn't take away from the fact that the Wizard's actual power, compared to a sorc, comes from the unique spells they get (esspecialy from level 5 upwards) like Bigby's Hand, Wall of Force, Contingency, Force Cage, Simulacrum, Clone ect. Every single one of those is pretty much a class feature by themselfs, not some situational tool or blasting spell #7.
I played with a Divine Soul Sorc when the Class Variant UA came out in a Storm Kings Thunder Campaign and tried it out. There were a couple of situations where it stood out to me:
-One was when we found a corpse near the end of the day. We set up camp and I learned Speak With Dead (one of those fun utility spells that no sorc ever has room for) for the next day. We learned some extra info on the hill giant's, buried the man and gained some reknown with the Order of the Gauntlet. It was a fun interaction.
-The next was when we heard about a wizard who set up shop in some sewers, was kidnapping people and needed dealing with. Going in I switched in Counterspel for the fight, a spell choice I had conciously skipped when levelling up because it's so situational (esspecialy in a Giants-campaign), and that was the first time I felt like I gained an actual edge because of the rule change. Joke was on me when it turned out to be an Aboleth and counterspel did absolutly nothing against it's psionics. I can still see situations where this would cause discusions at a table though.
-The last was when we were going after one of the Giant Lords and we ended up with the Fire Giants Lair. Kind of rough when your shtick is being a Tiefling fire sorc who picked up Flames of Phelegos instead of Elemental adept: Fire...So I was kinda happy that I could switch out my Scorching Rays/ Fireball ect to more appropriate spells and actually play my character during a very large climactic part of the campaign.
Overall I think it added to the gameplay although it could probably could use some tuning (perhaps the hit die-idea, perhaps something else) instead of just flat Spell Versatility and even then it likely isn't for everyone.