I have had success combining sites and also splitting up sites. It depends on the situation and your long term goals.
Does the main domain need help? If it needs help can you boost the main domain using other methods like providing free tools, adding more "evergreen" content, improving usability or some other method. Will you lose any backlinks that may become redundant by moving the blog into the main site? How strong are the usage signals for the blog? Do you have the time to deal with moving the blog and dealing with redirects and broken links?
I agree with goodroi to an extent, taking these considerations on whether going through with the change.
I have always found better success with subfolders rather than subdomains. And depending on your server you could pretty easily redirect the subdomain to the new subfolder. Again-taking the previously mentioned points into consideration
I was thinking about a faster way to boost the main domain, even we continue doing other things as you mentioned. In our case moving the blog to a subfolder is not a problem but I fear that could be for nothing.
The blog was created to support the main site, I think it could make sense, but only if Google doesn't treat already the subdomain as a subfolder in this particular case.