The main gateline is horribly congested, even at busy leisure times, not just in the commuter peak. The new entrance is a spectacular waste of money and does not address the crowding problem, which is here and now.
There is no significant crowding issue at the main gateline except at peak commuter times (or occasionally after a Leeds United home match). What crowding there is could easily be solved by getting rid of Burger King, which I believe is what is happening.
The main business (as opposed to shopping) district has moved southwards to around Bridgewater Place and Brewery Wharf. Leeds' biggest businesses are now largely south of the river. We're not talking about 15 years in the future, we're talking about the here and now.
As for shopping, it's the same distance from platforms 15/16/17 to the shops at Trinity whether you go via the main gateline or the new entrance. The Dark Arches should be more appealing once the refurbishment is completed, but I'll be honest, I've never had any issues either in the Dark Arches or along Neville Street, and I've been going to The Hop for years.
Full disclaimer: I quite like the new entrance because it saves me ten minutes getting from The Hop to the station, that's time for another half of Yorkshire Blonde
The distance along New Station Street from the main 60s concourse at M&S to the steel gate on platform 8 is about 50 yards.