Insofar I can ascertain from the AppleCare+ monthly policy effective in the UK at the time of your purchase, as I read it, the policy life is effectively limited by clause 3.2.2. When the covered model ceases to be supported for repair, due to unavailability of parts, the policy ends.
3.2.2 - Monthly Policy: Your Policy duration is one (1) month. Your Policy will automatically renew each
month beginning from the date You purchase Your first Monthly Policy as reflected on the original
sales receipt for Your Policy. In the event Apple is no longer able to service Your Covered
Equipment, Apple will provide You with one month advance written notice of cancellation, or as
otherwise required by law, after which Your Policy will cease to renew.
Had the iPad been exchanged during its support lifetime, the current monthly policy would carry forwards to the replacement. As the vintage iPad was exchanged due to unavailability, the existing monthly policy ceased - requiring a new monthly policy at the current rate - the increased cost of which appears to be the basis of your complaint here.
Given the age of your [now] ineffective AppleCare+ policy and the hardware upgrade that you have received through replacement, the new monthly AppleCare+ policy arguably still represents good value for money given the peace of mind that you seemingly derive from it.
Contributors here are all end-users, just like you. We have absolutely no influence over Apple or its policies - or its Insurance Policies (in this case administered by AIG) which differ by region/country. Other than the Moderation Team, Apple generally do not monitor or act upon posts appearing here.
If you have concerns over the legal status or applicability of the AppleCare+ Insurance Policies that you currently have, you need to seek qualified consumer and/or legal advice - starting perhaps with your local CAB, the UK Financial Ombudsman Service (FOS).