Many people wouldn't be able to walk that in 36 minutes either - would you propose telling them they're not allowed to walk, or else increasing the MCT in case they do decide to walk?
I just don't see that as an issue. Just say your ticket will be accepted if you want to make a cross-London transfer by bus, but it's 'at your own risk' in terms of timing of connections. TfL already excludes buses from its delay refund scheme, so it's hardly a stretch to say we're not going to be responsible for getting you between terminals by bus in any particular timescale, is it? I don't think anybody's proposing that National Rail journey planners should start telling people to make transfers by bus, are they?
Having said that, some NR stations in central London are quite close by bus but not by tube. On a journey from Hertfordshire to Surrey, Thameslink to City Thameslink, bus to Waterloo is a perfectly sensible route, for instance. There might be a case for defining fixed bus links between certain station pairs, such as those two, for the purpose of allowing cross-London transfers to be made by bus between them. But I'd have thought the fixed links that exist already should continue to be based on tube transfer times, and people should be told if they choose to get the bus and miss their train, that's on them.