Atleast DJ makes sense now. That bit didn't make sense to me for years, straight asspull until WCI.
As for your question, Oda already ruined Sanji's character on WCI. Now the least he can do is abuse his genetics to keep him relevant as one of the future PK's top fighters. Last chapter did nothing that WCI already didn't do, in fact it would be an utter waste to make Sanji go through all of that nonsense on WCI and not reap any of the benefits.
This was a power level move by Oda, this is for all those people who say Oda doesn't care for power levels, another example proving he clearly does. He needed a believable reason for Sanji keeping up with the top hitters of the New World, why would a fruitless cook who spends most his time cooking be naturally keeping up with Zoro who only has 1 purpose. As bad as people think this is, I certainly like it more than Sanji's haki and AP being in the realm of other non DF monsters. That takes away from the likes of Zoro, Mihawk, Shanks and especially Garp, for a cook who isn't even the second strongest in his own crew to achieve those things at such a rapid pace, diminishes their accomplishments. Oda could have easily just had him magically keep up with the heavy hitters naturally but he's clearly trying to gatekeep that level without a devil fruit (and it makes sense, you might as well throw a df in the trashbin if the likes of Sanji can achieve such feats at such an early age), from a thematic pov this might look like the wrong choice, Oda chose logical Power levels > story/narrative in this case.