Speaking about the UK, so apologies if the situation was different elsewhere.
But...
From how I remember it, these 'respectable national couriers' were just not doing the job they were paid to do.
There was a 3 month phase when I didn't recieve a single next day delivery on the next day.
It is a very rare occurance now when I don't get an amazon package delivered on time through amazon logistics.
As soon as one company switches to self employed 'gig economy' compensation for their fleet, they get a big advantage in costs which means they can push prices lower.
The only way the competitors can match it IMO is by also switching to the same model, otherwise they lose contracts to the companies that do.
I think it's especially difficult in parcel delivery as the 'customer' is the store you order from, not yourself. So you don't really ever buy parcel delivery services yourself, it's always subcontracted.
Not really sure the best way to solve this apart from govt regulation, but feel it would cause all kinds of crazy side effects if not well thought out.