The fall of the Tory party

That's the obvious response, yes. Corbyn as proof of progressive policies not being accepted in the UK more recently, and a majority of countries in the OECD whose left wing governments are relatively centrist, if they're even in power at all. There are however outliers both domestically in the 20th century as well as current international examples.

Here's part of Harold Wilson's manifesto in 1974 before a narrow victory:

To that end, urgent action is needed to tackle rising prices; to strike at the roots of the worst poverty; to make the country demonstrably a much fairer place to live in. For these purposes, a new Labour Government, in its first period of office, will:
[...]
Redistribute income and wealth. We shall introduce an annual Wealth Tax on the rich; bring in a new tax on major transfers of personal wealth; heavily tax speculation in property - including a new tax on property companies

In 1970—an election he lost—he laid the ground work for those arguments with the likes of his 'Selsdon Man' speech:

What they are planning is a wanton, calculated and deliberate return to greater inequality. The new Tory slogan is: back to the free for all. A free for all in place of the welfare state. A free for all market in labour, in housing, in the social services. They seek to replace the compassionate society with the ruthless, pushing society. The message to the British people would be simple. And brutal. It would say: ‘You’re out on your own.’

Left wing: yes, won: yes. Left wing: yes, won: no. Different factors, a different time, and a very different left movement, to be sure.

Today's overton window is in a very different position, and most recently Corbyn set the boundary for electability to his right. Given the polls right now it's likely that the threshold of electoral viability is to the left of Starmer, or otherwise down to factors external to Labour policy, like Conservative incompetence and all the rest that's discussed at length in this thread.

So with all that said, do you honestly believe that there's no room whatsoever for more progressivism in the current Labour party that could be electorally viable? How about just a smidge more progressive? Two smidges?

It's quite obviously a grey area. It has some risk, sure, but it does provide space.