more to it than that.
if the cylinder is acting as "heat sink" and you close a valve, where will the heat go? A 100l cylinder can absorb a lot of heat before it boils. There should be an always on rad, but even so, a risk you would boil in the back boiler, might spurt out of an overflow, might be squirting out in the loft and softening the header tank, might damage the boiler if it boiled dry, and soldered joints might get hot enough to melt. I have an idea some old ranges were built with a fusible plug that would melt, allowing water to escape into the fire and extinguish it.
Thermostatic cylinder valves like Cyltrol and Tapstat are (were?) expensive and not widely sold in recent years. I had a NOS spare that I sold on ebay about five years ago after selling an old house, I think it went for about £80.