you can build a supporting case for ultralights taking over GA and that may well happen but if it does it will kill off most GA airfields, which will confine flying to private strips
IMHO it's inevitable that the luxury of flying off vast expanses of (usually ex-mil) hard runway at low cost will go. The traditional 3 runway layout uses up a lot of real estate. Those that are suitable are gradually being taken over by the expansion of the LOCO's into regional airports and those that are not are under pressure to be redeveloped to provide greater returns for their owners.
A 2 runway layout like Popham or a single runway layout like Old Sarum or Sandown uses up a lot less space.
Those who choose to fly a/c that need runway lighting, paved runways and approach aids will find their choices limited to developed a/d that have commercial traffic to help pay. Those who don't need such things will vote with their wallets and move to places with less grandiose ideas.
I'm no great fan of ultralights. They exist as a class only because the regulators have created the classification, some indeed are available in versions that fall into more than one class. If an aeroplane is airworthy it is airworthy. The airframe, the air through which it flies, and the powerplant that pulls it along don't behave differently depending on whether it is on a Permit or C of A, whether there's a built-up area underneath, whether it's dark outside or whether the pilot is flying by reference to instruments.
Having re-read that paragraph I'll rephrase it. I'm no great fan of paring away a structure simply to force it under an artificially imposed weight limit. While paring the weight down inevitably improves performance it also results in structures that may have little margin of strength above the absolute minimum called for.
Mike