I agree transport is key- without good links to the rest of the country, most businesses don't want to move to an area. It also helps if buildings are there ready for them to move into and obviously these days a good internet connection is a must.
But I do agree for ambitious young people with a partner who also works, one employer towns do not look like great options and I'm not sure that creating one big employer to "save" each town is a long term viable answer. What happens if that one company goes under?
If we want to save small towns, I think part of the answer will be companies allowing a lot more working from home with perhaps coming to a central office one or twice a week. But obviously there are many jobs this would not work for.
However, it's not that easy to just up and leave- I say this as someone who has re-located across the UK for work on more than one occasion, and many of my friends have too. When you rent, it is bloody hard getting together a deposit + first months rent +agency fees +moving costs. For a 1 bed flat costing £500 a month, you are looking at at least £1500 in many cases (£750 deposit, first month's rent up front, £250 agency and referencing fees, plus £250 moving costs or to furnish the place). If you are unemployed and on benefits, that is out of reach.
If you wanted to improve prospects of young people, you would offer them this as a grant if they had a job lined up in another area of the country, or maybe a loan to be payed back on similar terms to student loans. You'd also offer help with costs of traveling to interviews. This would ultimately kill some towns, but it would help the people, which are surely more important than the place?
Also, I do think seasonal and temporary work has to be a better option than staying on benefits, and benefits should be flexible to support employment on a more "as and when" basis. Right now, if I was reliant on universal credit, I would think taking a low paid seasonal job (for example) was a big risk.