There are many many reasons you don't want to (or can't) have the apex of your domain (what you're calling the "default domain") the primary domain name of your site. I thought you were going to argue in favor of like "home.[domain].com" or something.
The first and foremost issue is that if you wish to use a CDN, many CDN's require a CNAME to function properly. You can't have a CNAME share space with any other record (RFC 1912) which completely precludes using an MX record (for email) or TXT records for DKIM, SPF, and DMARC. (You need those for a secure email service). Having the CNAME sit as a subdomain of the apex (such as www) allows you to maintain those records in your domain while also serving people using that CNAME. (Some CDN's such as Akamai even have special proprietary records that function like a CNAME while returning A records just to make this work)
Warning, the below is not uplifting @ all. Avoid if you don't wanna see it.
spoiler
Then there's my parents who disowned my younger brother after he came out as male.... They kicked him out of the house and now he's broke and alone trying to scrape by in the world. I'm one of two family members that supports him out of a previously huge family network. I tried to warn him that this would happen but he was convinced that everyone would do a 180 on their deep-rooted political beliefs.