I don't know, this sounds a little bit like that sovereign citizen "wrong flag in the courtroom" stuff.
Even if they haven't replied, the original contract is still sitting there clearly stating that a certain set of assumptions are in place if you use the software. I don't see any way to argue that a non-reply implies consent to new terms or abandonment of any terms.
Re:Why I Don't Sign Non-Competes
by jonathanstrange on Thursday March 22, 2018 @06:00AM
(#16647240)
A contract is only valid if both parties agree. If one of them explicitly disagrees, the contract is definitely not valid.
Send a letter via snail mail that clearly states that you disagree with their EULA but were unfortunately only made aware of the EULA after your purchase. Include a printed, revised EULA with your proposals for changes. Be reasonable in the changes you propose.
If they reply to you that they disagree with the changes, they can prohibit you from using the software or service, but then you're also entitled to a full refund, of course.
If they forget to reply, their EULA will not be enforceable.