Well, without the transfer fee the club could get for them that chance would be even smaller. I wouldn't want Arsenal to make really bad financial decision just to please the fans. There wouldn't be much of an argument for Arsenal to spend real money on the team if you'd expect them to leave good money on the table for no good reason at all.
i mean if you really want to make the claim that neither of them is going to be worth more than 20m then you're setting the whole argument up to be agreed with. I myself am expecting them to be worth much closer to what the club paid for them, so somewhere in the range of 35-40m. On how long the club usually takes to replace them, that's a fair point, though there is also the question mark over if that wouldn't just be delaying the inevitable for another year.
Honestly any less than 60m for the pair of them, even if only on one year, would be daylight robbery by pretty much any club these days.
I guess it's the difference in valuation which is the main difference here. I wouldn't bother selling something perfectly good either for no good reason if I was only expecting half of the money I should really be getting.