Firstly, We don't CURRENTLY use them because we have better options in the squad - that won't ALWAYS be the case. Distin, for instance, will eventually retire and Duffy, on previous performances, might be earmarked as his replacement. Vis a vis Jagielka and Stones. That's why teams develop younger players. And secondly, no-one wants them mate.
We could get their wages off the books by shifting them at firesale prices, but what kind of a fund would that generate for the transfer fee itself? What price do you realistically see Gueye fetching? Or Duffy? If the rumours have any substance, we've been trying to shift Heitinga for over a year but no-one wants him. I doubt you'd currently get £5m for all three combined, and what sort of wages do you think Gueye and Duffy are on? I don't think they're on a significant wedge, compared to Heitinga. You might free up £150k per week, max. And incidentally, I think we'd be well advised to keep hold of Duffy - our defence is not a youthful group - and Vellios, who we often forget is still U21.
Alternatively, if we hold out for a decent price for Fellaini we could generate a much bigger transfer fund (I have stated over and over that any bid less than £30m should be rejected), and remove Felli's £75k a week from the payroll. I'd have no quarrel with shifting Gueye and Heitinga in addition, to free up maybe another £100k a week off the payroll, but the idea that this would generate anything more than about £3m in transfer fees is fanciful.