Or customers will switch to a competitor's API that doesn't keep trashing their app with "improvements' that are not usable for that specific app. But your pro

Or customers will switch to a competitor's API that doesn't keep trashing their app with "improvements' that are not usable for that specific app. But your proposal is interesting: if things are improving all the time, API-users need to be upgrading all the time and adapting to the new features anyway - so any incompatibility necessary for those features is no problem. It depends on the phase of market (new, growth, mature, stagnating) for that specific API. At the moment, most web-APIs usages are "new".