It's difficult to think about what's happened to high streets in isolation from the question about what's happened to our towns, suburbs and shopping/socialising habits.
I'm not sure why it's a surprise that high street shops are closing at the same time as we continue to build large, attractive, enclosed shopping malls with pubs, cinemas, restaurants and free parking. People obviously prefer to go to those. The same people then express sadness that their high street is not full of quaint local butchers, grocers, florists etc, but the simple reason is that the people doing the complaining often never gave them their business anyway, they just want it to look nice and quaint. So the natural result is that the out of town shopping centres are visited by the out of town and suburb dwellers spending lots of cash and getting good quality and choice. Not to mention the huge growth of convenient online shopping, with possibly an even bigger effect on the high street than the malls have.
Consequently the high streets are left empty, or filled with betting and charity shops, interspersed with rough sleepers and graffiti.