"Enormously more expensive" is just wrong.
That may be true for certain specific cases, depending on "how the cookie crumbles". In that case, he'd be able to give you specific reasons why that technology would not be wise for your house.
One example might be a need to go with a non-direct-vent gas unit in a new location and have to punch a hole in the roof for new venting.
If he's making that as a blanket statement, he is just plain wrong. I would suspect an ulterior motive, and you probably don't want to start that conversation LOL.
Electric on-demand heaters are not necessarily more expensive than tanked heaters. I generally find they're surprisingly inexpensive. Gas on-demand heaters are pricey, however, especially if new vent plumbing is required.
New technology works better when you're smart.
Many new technologies are vulnerable to being applied in a klutzy and inefficient way that makes them not work well at all. That's where, again, skills pay the bills - and people without skills get frustrated trying to make new technologies work.
One example of a sizing issue: Adding a low-flow showerhead can allow use of a much smaller heater, which can mean running much smaller cable.
With on-demand, you might use more
This means the middle person in the shower who used to have to hurry to leave the last person anything at all, doesn't hurry anymore. The last person can take the time they need. That extra use is a true luxury you now have, but it's not free.
Crossing over from gas to electric may add energy cost
Since electric energy starts as heat, and only 33-40% makes it into electric energy, it's an inefficient way to buy heat. When you burn your own natural gas, you get 80-90% of the therms in your water. So, X therms of electric is generally more expensive than X therms of gas.
It's not only about cost: convenience is worth it.
We are affluent Americans. We have money for conveniences. On-demand does some amazing things:
Unlimited hot water. This is huge. Especially if you have a family showering.
Quick hot water. With electric, you don't need a vent, and the unit is not large, so you can position it near the point-of-use. That means hot water in seconds, not minutes. What's your time worth? What's the water you flush down the drain worth?