Sure it can be accomplished. Some thoughts:

If the slab is post-tension, that's an issue. Cutting into slabs with cables is a serious consideration. If not then move on...

At a minimum, the shower footprint section of the slab will have to be cut out and removed so the shower elevation can be lowered to achieve a curbless entry. That in and of itself is not difficult.

The drain itself and the branch line connecting the drain to the waste line? Right now the tub drain itself is at a certain elevation and the tub trap and branch is pitched for drainage into the main waste line. If you're dropping the elevation of that trap down a few inches to achieve a curbless entry, then the entire branch line itself may have to be dug up and repitched.

There's also the diameter of the existing tub branch, and if it'll have to be resized to accommodate the required diameter for a shower branch line. Having to resize the branch could be a big mess.

And it also depends on how deep the tub branch line is buried...it may be well below the slab, which works in your favor. Or it might be right below the slab, or a part of it might actually be within the depth of the slab. The deeper the branch line it buried, the more it works in your favor. It gives you more vertical elevation to play with at the shower drain/trap.

Without cutting into any concrete, the waste pipe at the tub stub out could be examined to see how far it goes down vertically into the slab and the earth below the slab before it runs horizontal.

They may find that they have enough vertical at the drain to play with so they can leave the branch line as is, pour a new slab, and move on with the curbless.

Or they may have no vertical to play with, which could turn the dream into a nightmare.

Those are probably the main concerns your contractor is worried about; the amount of vertical he has to play with at the drain, and if the branch line has to be resized.

If it's something you truly want to pursue, then you need to talk through the ramifications or moving forward with your contractor. One, to make sure your contractor isn't simply saying "no" because it's the easiest answer. And two, that you know you're opening a can of worms, you know the ramifications of moving forward, and that you're willing to pay the contractor to do the extra work that may pay off in the end with a curbless, or it may all be for naught.

"Anything" can be done. But it might not be able to be done within your stated budget. Or perhaps it's not within the skill set, the level of knowledge, or maybe even the comfort level of your contractor.

Anything can be done with a robust enough budget. Is a 15K-45K shower within your budget? That is just the shower portion. The rest of the bath would be on top of that. Just a well waterproofed shower is 5-7K for nothing spectacular. A curbless on a slab where everything is "easy" and goes correctly adds another 10K on top of that. Jackhammering all of the plumbing in the house will take that $$ to the stratosphere.

It is alot of work........I have built more than a few as we have a lot of low overhead basements here in My locale... Hire wisely.

Basement recessed curbless tile shower info here

I want to thank all of you for taking the time to respond to me. Very, very helpful.