My statement means exactly that.
This year 5 and 5c support would be dropped
Next year 6 and 6 Plus wouldn't receive iOS 12.
Nah I don't think that'll be the case. Besides you're missing off the 5S. If any would be dropped with iOS 12, it would be the 5S, but I don't think that would happen either as it has similar hardware performance to the 6/6 Plus. And there's still the iPhone SE which has the 4" form factor, which is obviously still being sold.
Apple historically support hardware and software updates for at least 5 years from the date of last manufactured. There are a few exceptions to this rule, though that's normally when the product was in its infancy and the older hardware just can't hack it. Hardware performance is considerably more capable since the iPhone 5 and the only reason the iPhone 5 is being dropped is because Apple want to move to pure 64-bit development & OS.
TL;DR: if support will be dropped, it won't be due to the RAM. Not in the near future, anyway.