Am looking at thankyou pages and came across this post which sums up why a thankyou page is so important in user experience. The thankyou page is not the end of the process – as many thankyou pages make out – but the start of engagement with the user.

If they got to a thankyou page the user has done something positive with you – signed up to an email, bought something etc – so this is the point at which you want the user to carry on engaging with you.

They might not want to carry on engaging at that point but at the very least give them the option.