The bath in our master bathroom hasn’t been used since we bought our house over two years ago. It’s going to be replaced by a walk in shower in about two months.
If we didn’t have room for both, I would do this too. I use my shower once or twice per day, the bath once or twice per year. (and I like baths–if the tub is big enough to make it comfortable).
Or maybe, you should look in the bathtub before you fill it with water?
Anyhow, I have a crummy little tub, and rarely use it, but I miss having a tub I can soak in. My husband uses tubs differently from me, and uses it at least weekly. I’d have replaced the tub with a larger one, but a structural engineer advised against it. 
Still, I think a big luxurious shower would be at least as strong a selling point as a bathtub, for the average buyer. And if that’s what YOU want, go for it.
I NEED a bathtub for soaking occasionally. I get horrendous cramps from time to time that can only be eased by a long tub soak followed by a long lie down with a heating pad after.
For an occasional deep soak, I’ll run the shower to do a basic scrub + hair wash, then I’ll soak in the tub for a bit & then briefly rinse off the suds by shower again before I’m done.
I have a jacuzzi tub in my master bath. I rarely use it.
When I remodelled most of the house a few years ago, I put a hot tub in the back yard. I use it several days a week.
I still need to remodel the master bath, but I’m likely to get rid of the tub. When I eventually sell the house, the hot tub will probably go with it. And I do have a standard bathtub in the other upstairs bathroom.
I love my proper, glass doored, Grohe equipped, travertine tiled shower. Ms. Fluffy likes her bathtub. To each their own. As others have said, I would be cautious about eliminating the only bathtub in a home, but if there is room for a roomy walk in shower, they sure beat the hell out of a tub shower combo.
Now about spa tubs - I would only recommend air tubs, Happy Lendervender’s cautionary tale well illustrating the very real and hard to fix flaw of jacuzzi/jet tubs. Crap gets in the plumbing … and yuck. It says something that Habitat for Humanity and other used building material stores will not take them for free even. that beeing said there are some truly wonderful soaker tubs available.