What are the ramifications of changing the Outlook 2010 default pst file?

As you mentioned, each email account has a corresponding data file by default which is used to store your Outlook data such as emails, calendars and others. If you change the default data file, the location to store the information of your mailbox will also be changed.

https://support.microsoft.com/en-us/help/983131/by-default-a-new-outlook-data-file-pst-is-created-in-outlook-2010-when

Outlook 2010 is at end of life and should be upgraded to a newer version. Updates to Outlook 2010 are just security updates (but that may have altered something about passwords). There are no feature updates to Outlook 2010 (Office 2010) to correct any issues. I do strongly suggest you upgrade to take advantage of features. I do not have to enter my password each time with Office (Outlook) 365 using POP3.

Each new email account makes a new PST file. That is default Outlook behavior, and I am very sure this was the was Outlook 2010 worked when I used it.

The Microsoft account is now the default PST file. which doesn't make any sense to me.

Microsoft Accounts were not around at the beginning of Outlook 2010. I think it became default because it was the newest account.

I'm just confused why there would be a default PST file.

Each new account makes its own PST file. You can do this, or you can use an existing PST when you make an account. I do the latter myself.

There is always a default account and you can change the default Account in Outlook Email Setup for Accounts. There is no issue if you change (A) to be default instead of (B)

The size of your email files is dependent on the mail and attachments you get, not on whether it is a Microsoft Account or the Default Account.

Here is a basic screen shot of an Account Setup. You can see here that my first account is the Default Account. I set it this way. I have all email going to the same PST file, but that is just personal choice and you can do this your own way.