I don't think 80% at an early stage is ever justified as it not only hurts future investors from considering you in future rounds, but it hurts founders.
I like PG's equity equation to calculate the value of investors. I don't think any investment can increase your odds of success by 400%.
I want to add on that my partner is right in the sense that having 20% but able to expand to overseas might be much more than having 70% locally. However I still feel that we should start our steps small then proceed forward.
If you want to give up control of your company and product this early and this cheap, you way as well sell 100% of it now. Also, are your prices in Euros, US Dollars or Yuan?
Anyway, keep saying NO while there's any uncertainty.
Selling 80% equity is effectively an exit. Just think if you are willing to sell a company for that much, and if so, go for it. Having no control over what remains will make you effectively hired managers working for 10% of profit each, so after that, what kind of salary they are willing to pay you will be your biggest concern. Making something which is easy to replicate doesn't put you in a good negotiation position for sure, so maybe just take this cash, then walk away, and use the money to fund yourselves for a couple more years to build something that isn't that easy to copy.