VM prefers to give introductory offers to new customers to draw them in. Once they have you on the hook they can up the charges accordingly until the customer is sufficiently motivated to leave.
Your suggestion is based on the (not unreasonable) assumption that VM might be keen to hang on to its existing customers but that doesn't seem to be the case and VM is happy for customers (sometimes very long-time customers) to disappear in the general churn and turnover of cancellations, renewals and new customers.
You have made the correct decision to leave if VM's offer is not competitive. There have been past topics on here where a cancellation prompts a call-back from VM with a much better offer (but there are no guarantees that any call back will actually happen).
Generally it seems to be that once you have left VM for 90+ days you become a 'new' customer once again (although timings for this can vary). Some people have got a partner to take out a new contract at the same address and benefited from new customer pricing that way (but that comes with some additional hassle of loss of existing VM email addresses, loss of an existing landline number, return of existing equipment, installation of new equipment, possible loss of service continuity during the switchover etc. etc.) so anyone considering that should look into all the implications of that route before committing.