In 99 cases out of 100 those fears are unfounded. Is this only a thought experiment, or have you experienced poor hot water delivery?
How many LINEAR feet of tubing are we talking? Even a thousand feet of 3/4" pipe still has only ~20 gallons of water in it, which isn't a huge amount of thermal mass 167lbs of water- it'll rise above showering temp pretty quickly even with only 60,000 BTU/hr ( 1000 BTU/ minute) of burner behind it. If the heating system water had fallen to 65F , rising to 115F that's a 50 F rise, for 50F x 167lbs= 8350 BTU. At 1000 BTU/minute it's less than 10 minutes before it's starting to put significant heat into the indirect tank, not a few hours.
What is the radiation type, and how much does it emit at an average water temperature of 120F? If big old fashioned radiators, how much water volume?
What is the DOE output of your boiler, in BTU/hr?
If your boiler's output is 75,000 BTU/hr, and the radiation can only emit 50,000 BTU/hr at 120F the remaining 25,000 BTU/hr would be going into a 110F tank. It'll take awhile to recover if somebody takes a long enough shower to bring the tank down to under a comfortable 105F showering temp, but the radiation will be emitting less at the lower temp too. It'll be no worse than an electric water heater.
Only if the radiation can emit more than 90% of the boiler output at 120F would it be an issue, but still not an hours long wait for hot water.
The more usual condition is that he radiation may be able to emit 100% of the DOE output at 180F entering water temp, but only about 35% of the output at 120F. So the indirect would still be taking the lion's share of the boiler output when the indirect temperature dropped to 105F.
With some real numbers on the tubing & radiation volume, heat emittance, and boiler output we can get a handle on just how close to the line you're skating, but it's pretty rare to fall short.