The problem Navien and any other manufacture's have to over come except Rinnai (who has done a great job) is service and has is to create a national service program.
When CEC imported and distributed Bosch, you couldn't ask for better technical support. When Bosch acquired CEC, they seemed to completely down play service, maybe not understanding the north american market. the fast development of new products, lack of parts and support killed them in their tracks, for now. the B series is bullet proof and no one has anything like Bosch's H1600. The 2400 and 2700 series are a mile stone and all Bosch has to do now is build confidence back because these are both superior units. Bosch's C800 condensing is also a worthy contender. Rheem's ECO line holds its own.
What matters at the end of the day is service and not just calling customer support but someone local who knows the product, has the experience and can take care of the problem and you just can't do this over the phone. When the customer has no hot water and you have to wait for parts to arrive, the customers experience with a tankless goes south in a heart beat.
If auto manufactures treated repair and service the way most tankless treat service, we'd be riding horses.
Who has the greatest influence on a purchase in my experience, the plumber, for better of worse. All manufactures need to increase training of installers and service personnel. If we were trained as plumbers the way manufactures train installers, we'd still be in the 18th century. I've sat in on too many late afternoon installation classes where the trainers have flown through the material and lost most after 5 minutes. Training should be a day long affair, not an hour or even two and include live fire, practice an installation and at least some limited trouble shooting to understand problems due to short sided installations.
Tankless is clearly hear to stay and the old excuse for using tanks is in the history books. Does anyone still install galvanized water piping, no! Are homes built with cast iron waste except in the second story, not many.
When I first started Plumbing in the 1970's, there were still lead closet flanges that had to be repaired, 7 gallon wall hung with 14" rough bowls were still in use. I remember working on a Rhuud water heater that was copper tank with rivets from the 20's with gas and air levers for adjustment.
I still have my roughly 60 caulking irons and asbestos Italian runners for lead work when I first trained and my gas torch and lead pot. Who would have thought of a fraction of the tools and materials now available much less micro chips on a water heater? To sit and deny what is changing is a fools death. Tankless is here and not going away and more manufactures will enter the market. How they treat plumbers and service personnel will influence who will be around at the end of the day...