Suppose this went through and VeriSign later engaged in .tv-style pricing for .com domains. Everyone would be aware that domain costs would skyrocket in 10 years (i.e. assuming you renewed to the max before the price increases).
That means in year 1, no rational person would pay more than 10 years income for your website/domain.
In year 2, no rational person would pay more than 9 years income for your website/domain.
In year 3, no rational person would pay more than 8 years income for your website/domain.
and so on. Basically, your business would be dead. Tiered pricing would be like the Mafia deciding they want to "partner up" with you, and they get to decide how much you pay them each year. Would anyone want to buy your business?
Think about Russia, and how the authorities expropriated companies from their rightful owners, and gave them to their friends. Would you want to invest in Russia in that climate?
Without certainty as to the renewal price in the future, the entire landscape for domain registrants would be put at risk. Allowing VeriSign and other gTLD operators to cherry-pick the most favourable terms to themselves from new gTLD contracts puts us all at risk. When there are 500+ registry operators, each of them putting in requests to modify their contracts on a regular basis, as they do now:
http://www.icann.org/en/registries/rsep/
this would create a lot more risk, unless procedures are put in place to stop it.