I wonder whether some people don't really understand how higher-pay type jobs work, and that's why they don't understand the consequence to the country of those workers being dis-incentivised from continuing to work, or choosing to leave the country.
For some jobs, it doesn't really matter too much who does it in terms of what gets done. The job might need some training, and you may
pay a bit more in order to attract good people and retain them after investing in their training. But nonetheless once you've trained that person up, it doesn't really matter whether they're eg a brilliant train driver or just an acceptable one (so long as you can get rid of actually unsafe ones). The brilliant driver can still only drive one train, with a set number of passengers. So even the most brilliant train driver can't demand more money than an average one - since you don't really care whether you have the best train driver or the 2nd best train driver. And you can always train another: it will cost, but there are lots of people available who are capable of learning to drive a train well enough.
jgw1's argument holds for this type of job.
Then you get the kind of job that pays the big bucks. These are the jobs where the individual's performance makes a genuine difference to the profit made by the company. Strong performers in management, law, IT etc can be many times more productive than weaker ones - and can also make others around them more productive too (which multiplies up the benefit they bring).
In this case, if the best performer makes the company £x more than the second best performer, it makes sense for the company to offer an extra £x-1 pay to get the best performer instead of the second best performer. That's why these jobs pay lots.
If the best performer decides to go part-time or move to another company, the company will make £x less money. They will have to pay the replacement £x less to reflect that. If the top performer moves abroad, then the country creates £x less value and the state receives less tax (both less income tax for the replacement and less corporation tax on the extra profit the company would have made).
By definition, for these types of jobs you can't just give the job to someone else for the same money and get the same value out of them (otherwise you already would have)