If a subdomain blog has high ranking from backlinks, it’s not really a problem that this doesn’t transfer to the root domain. The visitor gets the content they want and the visitor is also exposed to the company/product that owns the blog. For example, if I land on Stripe’s blog and I’m interested in a credit card processor, there’s a high likelyhood I will also visit their main site, and I will be more qualified since they’ve already provided value from the blog post.
Search engines rank pages, not domains. Search results aren’t a list of domains, they are a list of pages. The domain is only one factor in the page’s rank. Whether a blog lives at a subdomain or root domain, the root domain will not rank higher than the blog post for keywords backlinked to that post.
"Make good content others want to link to" is not something automated SEO testing and SEO consultants can sell, unlike "You have a subdomain blog, please move it to /blog for better SEO points."
The problem is when you want to rank both your content on the blog AND some dedicated landing pages. Keeping blog separate will make it harder to get the juice passed to your landings.
The assertion that sub domains don’t contribute to domain authority is not backed by facts or authoritative evidence.