To be fair, I think it’s worth clarifying that there are three separate flaws here, in the reasoning of people who want free continuation of email after switching ISP...
1) They can’t count on it being free. It’ll cost the old ISP money, even if just peanuts, and the old ISP might want to recover that cost, which is not unreasonable.
2) They can’t count on it, full stop, even as a paid service. Email addresses depend upon a domain name, and domain names are transient. As I mentioned earlier, even Google might one day stop renewing ‘gmail.com’. Might sound unlikely but all companies come and go, and Alphabet will eventually go.
3) They have Ofcom on their side, which they think will prove them right. It won’t, Ofcom seem to have no skills or understanding whatsoever in the technology of telecommunications, especially internet access. Just like companies, government agencies come and go.