You can make a ton of money a week just doing tanks but in my area the tankers are known as the N*****s of the plumbing fraternity.
Insulation didn't beef up in tanks until the FVIR system came along with it. Wholesalers bought literally a million extra tanks the year before FVIR came in just to avoid it as long as possible.
NH you're familiar with tankless if you're using the Buderus GB142 with a Priority circuit for DHW so don't shame tankless.
Show me a tank capable of producing 250 gals per hour?
And you Combi criers.....radiant despite it's real efficiency and comfort will always be backseat to FA till people give up CA/C. Paying for Ducts and tubes isn't rational in a less than luxury house.
And as to Market penetration it comes with Marketing. Tankless had it so easy coming ashore in CA they have no idea how to market to the rest of America. I deal daily with Noritz, Takagi, Rinnai, Navien, Eternal, Rheem/ECO, Bosch and most of their RSM's and Mfrs Reps are clueless.
I reinstall 100's of units a year installed by DIYers, handyman and, yes, fellow licensed Master Journeyman Plumbers who miss or screw up important installation requirements.
New rules or ideas have never been welcomed into plumbing; FVIR omg no, indoor plumbing, Mr Rudd's Water Heater,
When you started installing tanks there were no 7 head human car washes flowing 17.5-22 GPM. New Technology.......I was installing tankless Burkay coil units in 1979 in conjunction with 300 gal storage to produce huge gph.
Cold water sandwiches can be overcome by about 3-4 methods every plumber should know or have been taught (I know them and there ain't no big red "S" on my chest).
In Japan in new building the water heater is universally piped and actually moves with the homeowner from residence to residence as they buy/sell their units. Except for the initial piping, plumbers are cut out of the loop there after in dealing with moving or replacement of water heater.
Jist is get familiar with the future. Feedback improvements to Mfrs and maybe, just maybe you'll stay in Plumbing or Fittings and not lose your way preaching the old tyme religion.
Yesterday ain't tomorrow and it sure as heck ain't next week.