As a person who was a BT employee and managed the BT Yahoo technical Helplesk through the late 1990s and into the early 2000s, I support your position.
The charges are unwarranted and vastly excessive, intended as an underhand method of having customers change their mind about switching provider.
My email address was set up for free 25 years ago. It wasn't linked to any paid service and wouldn't have been until charges started being applied. To that point I had two email addresses with BT, the one I used and the other that was automatically generated with the paid internet service.
Costs to BT in terms of email addresses are minimal, due to advertising revenue, and the block service agreement contract with Yahoo.
You can compare it to car rental or whatever, the fact is the charge is excessive, and has an underlying, underhand strategy behind it.
In terms of mobile number comparrison, the technologies may be different, however the issue of an email address being as important as mobile number consistency has been raised and is on the Ofcom radar.
What about other ISPs who delete email address, or charge to keep them?
This is not a specific BTinternet issue. If Ofcom get involved with this, then it will be a UK wide issue.
Surely if you don't like the price of something you don't buy it ? BT are a business and out to make a profit, there are hundreds of alternative email providers, some of which are free, so what's unfair when you have choices ?
You can switch without fuss, you're wanting to switch from BT without fuss but still keep perks you previously had, those perks were for being a BT customer.
The emails were given free as a broadband customer, if you no longer have BT broadband then your services that were free become chargable, when my phone was with o2 I got o2 priority deals, now I no longer use o2 I lost these perks, I fully expected to.
In terms of cost BT have to support the product, you'll have multiple advisors providing support, those advisors need wages paid, taxes like NI contributions, holiday pay, pensions etc
The domains need paid for, the servers that host the emails need paid for, the SSL certificates need paid for, email specifications also change, so again BT employ people to keep the service updated and secure.
As has already been pointed out BT are not the only company to charge for email if you are no longer a BT customer.
As for your comments " Costs to BT in terms of email addresses are minimal, due to advertising revenue, and the block service agreement contract with Yahoo." leaving aside that BT have moved some of their email service away from Yahoo to another provider, regardless of how minimal this cost may be or how it is covered by BT and the other ISPs they are in the business to make money so it would appear fair to me that they charge non BT customers for their email if that is what they want to do.
Having an email address with your ISP is not compulsory, it is the users choice to have one. There are plenty free and paid for email services that are not tied to an ISP and given that "we" are encouraged to shop around for the best broadband deals it makes sense to use a non ISP email account which negates the whole problem you are complaining about.
If you managed the BT Yahoo technical Helpdesk you will be well aware that the BTYahoo email system is poor and not worth paying for and that the technical support helpdesk is useless for anything other than very basic scripted responses so I can not imagine why, assuming you still have a BTYahoo email account, you would want to retain it let alone pay for the privilege. It may be a pain changing to a non ISP email account but you would only need to do it once.
EDIT: I should have added that BT do tell you in the Terms & Conditions that you will have read when you signed up to BT, that your email account will be deleted should you leave BT Broadband unless you take out BT Premium email.
See Terms and Conditions Sect 8