I’m going to be a slight bit contrarian… I wouldn’t buy a house without a nice tub. I’m a bath type of person (when I have time). And if the house has more than one bathroom, I’d love to have a tub in my own bathroom, as opposed to using the tub in the “kids” or “guest” bathroom. Kids can be pretty gross.
That said, I guess it wouldn’t be a deal breaker if there wasn’t a tub in the master bath. As long as there’s one tub, we’re ok.
As someone in the market for a house, I would be turned off by a house that only had a shower(s) and no tub. But not as turned off as having one or more tubs without a shower - I’ve seen the latter multiple times and it baffles me. I mean, we had a clawfoot tub in the house I grew up in, but it also had a showerhead, ffs.
I wouldn’t buy a house without a tub.
Make the master shower big enough for two. 
Consenting adults gotta have fun in the bath too.
Several years ago my parents remodeled their house and very much designed it for their tastes, resellability be damned. The original house design had a standard bathroom with a tub & shower combo with a small half bath on the other end of the house next to the garage. When the remodel was done the half bath had been moved and turned into a mud room and the master bath completely gutted and enlarged, with a huge step-in shower with dual shower heads on both ends and a bench in one corner. There is no tub in the house at all.
The house is on about 7 acres of riverfront property, with the main living areas on the river side of the house. That side has lots of big picture windows overlooking the river and two big back decks for entertaining.
While I don’t know shit about real estate buying trends I’d think that, for any potential buyers, the location and the basic layout of the house with the wonderful river views would negate any negative element the lack of a bathtub may have.
Also, anyone able to afford that house could probably remodel the bathrooms, and any other part, to suit themselves without too much trouble.
This is pretty much my opinion, but you might want to discuss w/ agents who have experience selling your type of home in your area.
We gutted our entire house, including the baths. We decided to keep 1 tub, just in the odd chance that one of us might ever want to take a bath, or if we happened to be babysitting a young grandkid who needed a bath.
But otherwise, our expectation is to be hauled out of our house in boxes, so we really don’t care about resale. That will (hopefully) be our kids’ problems. As a result, we did not convert either of our upstairs baths into a master (attached to a bedroom), even tho we easily could have. We set things up exactly how we wanted them, and taking a few steps down a hall did not matter to us.
I think the idea of an updated master with a nice, large shower is appealing - especially if there is another tub elsewhere in the house.
As you get older, getting the legs over the tub gets more interesting every year.
Sounds like the better move is to expand the shower. To be clear, the house will continue to have a tub, just not in the master bath. The master bath is the one off the master bedroom, and the other bath is accessible to all the other rooms without having to go through the master bedroom.
The main factor now will be cost. It will be very expensive to remove the tub and expand the shower. The toilet will need to move to the other side of the room which is a 3" pipe and not cheap to move. But I’ll get estimates for both designs.
I’m not sure how much longer I’ll be in this house. My youngest is in her freshman year of high school and I’d like to stay in the house until she graduates. But… well, not sure how much longer the marriage will last. I really don’t see keeping the house beyond about 5 years, regardless of how everything else shakes out.