Unless you want junior doctors emigrating or working as locums to make a decent living it was necessary. It's still a lot less than they have lost over the years so no guarantee they'll accept it.
As for the funding - 20-40% of it goes back to the government in tax, a further 9% goes on paying back student loans. Then you save on some of the locum bills because you've kept some junior doctors and those savings are considerable. Then you get people back to work who havent been able to work because of the delays in treatment. And after all that they spend their money and you get some extra back in tax.
The net cost is very considerably less than you think.