'And none of the leftists has come up with a satisfactory answer as to why most of the boomtime jobs went to immigrants.'
I wonder if you'd think so highly of all these 'immigrants' if you realised that if they are EU/EEA they can and do claim tax credits and child benefit on children who have never set foot in the UK.
Here's why the jobs went to them.
Because if they had children, many of them were able to leave them behind in their home country and pay a rogue landlord, who was breaking the law by letting out to more than is legal in one abode, letting out the garden shed, etc. for space to put a sleeping bag.
They were then able to work as many hours, when and how they could, and spent as little as possible in the UK, instead sending it back to their home country. No childcare issues, no need for housing over little more than a covered camping site.
Employers twigged they could pay such people under min wage even, and so would only hire them and not UK people, who wanted min wage and safe conditions.
But alas, something tells me you won't be speaking so highly of immigrants once the UK has to fully open its borders to Romania and Bulgaria . . . 