I disagree, populism is in the rise and Corbyn was is in line with public opinion on a few issues. Namely on taxation and nationalisation, where the public is generally left wing. Labour’s strategy in 2015 was totally flawed because it basically offered diluted Toryism and was debating on Tory ground, such as deficit reduction. Frankly, a left wing Labour Party has, at the very least, shifted the parameters of debate more to the left.
Besides, the 2017 election has cemented Corbyn’s position as leader since Labour performed well above expectations and won a few seats it typically only wins when it wins an election (Leamington and North Warks and a seat in Plymouth) and seats it usually wouldn’t have a hope in winning (Kensington & Chelsea and Canterbury, the latter has been a seat since the 14th century and has never voted Labour before 2017).