The whole second home/holiday home issue is a symptom of a big problem but it is not a cause.
The main problem is that globalisation has totally put paid to the rural economy. 40 years ago people in rural areas got similar jobs to everyone else, they worked in banks, post offices, local independent shops and manufacturing businesses.
My in laws used to have a shop in a market town, it sold all sorts of clothes from country working clothes, everyday clothes, smart clothes, school uniform, socks, underwear everything - if you could wear it they sold it. They also had a work room which did tailoring and alterations. They employed around 20 people at their busiest.
Their business simply couldn't survive the modern retail world. Their's, and many other businesses have been pushed out first by the big national chains and now by online shopping. In smaller towns banks have closed, post offices have closed, cottage hospitals and doctors surgeries have closed, schools have closed or merged, libraries have closed and there are just not enough well paid jobs around any more. My DH used to work for a electronics company in the nearest town, that closed down 20 years ago as the manufacturing was outsourced to the Far East, and that happened to many other companies too.
Younger people move away to find jobs and the whole heart of the community changes. A fair number of second homes were originally family homes, the elderly parents have died or downsized and the adult children want to keep the house, their childhood home, for holidays.
This has obviously happened in other regions too, particularly areas which had heavy industry and mining, all that industry has disappeared and those well paid jobs have never been replaced.
The regions of the U.K. have just been forgotten, large swathes of the country have had no economic investment for decades - the left behinds.
In attractive areas tourism has filled the gap and so holiday home ownership increased and pushed up house prices way beyond what most low paid local people can afford.
Providing more well paid jobs in rural areas would do a lot to stop this problem.