What evidence? What evidence that 'political bias' is the reason for loss of BBC viewers?
FWIW I think the BBC is biased and is not neutral. It has always been that way. It favours the political and business establishment. It rarely reflects the views and priorities of trades unions, gig workers, people dependent on benefits, immigrants or sections of society that are not represented or championed by those in power. It takes its news agenda from the Tory print media.
That is why it tends towards the government of the day - with its hands on the corporation purse strings - unless that government has completely lost the plot (like this one!).
But I have seen no hard evidence that 'political bias' is the reason for reduced viewers. Anecdotal stuff, yes - with fans of Brexit, Trump and Rwanda spluttering over their Daily Telegraph - but nothing that stands up as evidence. If there is plenty - what is it?