They're not going to put hottubs in the base game for a simple reason -- that would devalue the gameplay items in stuffpacks. Say they put hottubs or saunas or oh, I don't know, that fishtank table from MLP (or MFP or whatever -- I don't own it) into the base game, then they would be losing customers. As long as there still are people who are willing to pay for a stuff pack that contains a premium item, it makes no sense to give it away for free.
IIRC, Sims 3 didn't put hot tubs into the base game, but there were several packs and store items which had them. I think that is a better solution. In Sims 4, there are updates to the DLCs which I think should happen, but that can happen without putting the update into the base game (apartments being allowed in all worlds, I'm talking about you). But I doubt that will happen.
