Legionella thrives on stagnation, so isolating any part of a potable water system for any appreciable time is an extremely bad idea.
But it doesn't thrive at the temperatures that most stagnant loops will be at. Perhaps that is why plumbing systems with rarely used branches are allowed by code and don't appear to be hurting people. But if there is evidence that drinking from the pipes or water heater in a cottage that has been unoccupied for months is a Legionella health risk, I'd like to review it. Almost very outbreak I've seen described involved heating and airborne delivery - mostly cooling towers. On the other hand, Legionella bacteria seem to be in many if not most water systems and even the aerator on a faucet or a shock absorber can allow them to grow. And some codes do require hot water tanks to be at 140F.