The green belt is NOT a protection for beautiful countryside it is an essentially arbitrary limit on development land supply - an urban growth boundary.

And removing it doesn't end protection for countryside, openness, heritage or environment.

We have over 200,000 hectares - about 15% of the countryside designated as Areas of Outstanding Natural Beauty https://www.gov.uk/guidance/areas-of-outstanding-natural-beauty-aonbs-designation-and-management

We have 10 English national parks that cover a further 10% of England.

There are thousands of protected ancient woodland and a register of veteran trees not in woodland. All have protection within the planning system https://www.gov.uk/guidance/ancient-woodland-and-veteran-trees-protection-surveys-licences

There are 4000 Sites of Special Scientific Interest (SSSIs) - 10% of our land area. All protected under the planning system

We have protections for sites of archaeological, heritage and ecological importance. Natural England is a consultee of thousands of planning applications

This includes ancient walls, hedgerows, historic field patterns, and traditional built agricultural buildings as well as obviously ancient archaeology or significant ecology. https://historicengland.org.uk/advice/hpg/consent/hedgerowregs/

Then there are thousands of listed buildings and England's 19 world heritage sites. All are actively protected in the planning system

Then we have thousands of locally designated wildlife or heritage protections - DEFRA has has wonderful database of this stuff.
https://magic.defra.gov.uk/ 

Add in protections and controls for historic battlefields, flood plains, locally protected landscapes, bird habitat and historic mineral extractions.
https://www.battlefieldstrust.com/page201.asp 

All of these are specific protections for things we decided were important and worthy of preservation and protection. The Green Belt is none of these things.

The Green Belt is littered with scrap yards and old quarries, with unused airfields and vast acres of low quality pasture and arable monoculture plus hundreds of former industrial sites. It simply exists to stop towns growing at all.

We have all the protections and regulations we need to protect England's open countryside without the green belt existing. What we wouldn't have is an arbitrary designation serving no purpose not properly covered by better framed regulations elsewhere in the planning system.