Still, it's not always worth it to go mod-con. If you get the total heat load low enough it's hard to rationalize the expense even WITH low-temp radiation. I never need more than 130F heating water at my place but:
A: "right sized" mod con for my actual design day heat load doesn't exist, and
B: The smallest mod-cons might still do OK for heating, but would be marginal for my peak DHW loads without a large indirect.
C: A mod-con would save me at most ~100 therms/year over running the whole shebang combi-mode with the $500 tankless (or was it only $400? I forget- but it was cheap) plumbed as a boiler, and a reverse indirect, with burner to spare on the DHW front. The fact that a tankless modulates and is very low mass makes it easy to design a system that maintains near-maximum operating efficiency on the tankless, especially when combined with the thermal mass of the reverse-indirect plumbed as a heating buffer (not a heating zone.)
Since I need to maintain an indirect at ~125-130F for DHW anyway, I'm already suffering that standby, and the financial benefits of going with lower radiation temps are marginal, it was easy to head toward the buffered combi solution, if not the mod-con. Until/unless NG gets a WHOLE lot more expensive I'm not likely to swap in a mod-con. But if/when it does, the system as-is runs in the condensing zone for majority of the burn time, and well into condensing on heavy DHW draws, but still over 100F. With a mod-con it would realistically get 90-92% efficiency as-is, and I could reconfigure it slightly and probably squeak another percent or two out of it in heating mode but again, the difference in annual cost even at 4x the current fuel price wouldn't necessarily make it worth the effort to make those changes.
But that's my situation- YMMV.
Still most homes in the US have design-day heat loads well under that of a tankless, and peak DHW loads well OVER what a perfectly sized boiler for heating will deliver. The answer to making them both maximally efficient is usually mass, and combining the thermal mass for both DHW and heating buffering is something that SHOULD be done more often that it is. Modulation has it's limits- the most you get out of residential sized mod-cons is about a 5/1 turn down ratio, (most are around 4/1). The real difference in LOAD is more than 5/1 on every day of the heating season, but with sufficient mass cycling losses can be minimized even with fairly oversized boilers.