Water Heater duplex puzzle - Tankless or Attic Gas?

[Robin W.]

I own a split duplex in Nashville TN that has two gas water heaters in the attic above each apartment bathroom. 

It was fine for 7 years, but one day the temperature dipped down so low, a burst pipe destroyed the unit. Thankfully we were insured and it was covered and restored to good as new, and the original water heater wasn't damaged, just the pipe. 

Now years later, this water heater is ready to die. It's been a long time question for me whether or not I should continue replacing the water heaters and ensuring proper insulation in the attic, or if I should relocate with a tankless water heater downstairs. 

I had a few plumbers come out, in addition to the folks who restored after the flooding, and all of them agreed that there is not enough square footage in either apartment to relocate the gas heaters unless I convert to a SFH and remodel, and no room in the crawlspace under the house (about 2 ft tall) to install a low boy UNDER the bathrooms.

I believe if i want to relocate from the attic, I have to do tankless in each unit, but I'm wondering if it's worth the cost.. or if I should just stay the route in the attic. This is a buy and hold. 

If anyone has any advice, or went through this themselves, please let me know! 

I also can't seem to find any plumbers who are familiar with tankless, which is concerning.