Not sure why you think it is odd that people feel differently about the reason for an increase?
Fixing the arena isn't necessary, so it is likely that people would be willing to forgo an arena upgrade to save money, even if the increase were the exact same as in the shavings/grain situation. People really don't have the option of cutting back on bedding and grain, generally...they'd be irate if you just started feeding less and mucking out stalls less frequently to keep rates the same. If they want the standard of care to remain the same, it is a lot easier to swallow increases that are linked to expense increases for the barn.
It's all about choices, and pretty dependent on the situation...but the message does seem pretty clear..."communicate with your actual clients in a fair and honest way before you make a decision that will result in their being expected to pay a significant increase."
Lifestyle coordinator for Zora, Spooky, Wolfgang and Warrior
So here's a thought... Do you have space to expand/build a few more stalls? You like the clientele you have now and there are more people wanting to board there. So, maybe look at spending the money on a barn/stall expansion instead that would generate more income to help support future maintenance/improvements.
I would probably not refoot the ring in the situation you describe. There's no significant benefit to either you or the existing, good, boarders. Maybe look at rolling it to smooth out the uneven spots if it is a turf ring.
Also, hay prices have increased segnificantly over the past few years, so if it has been three years since your last increase, I would add a $20/mo increase to all boarders. This money could then be set aside in a separate maintenance/improvement fund for future projects.
Also, what is the size of your ring and is there room to make it larger? If it's 60'x60' or something close to that I definitely would not refoot, but might consider a ring expansion (and subsequent new footing) in the future. Anything less than 60'x 120' (or close) and I find myself preferring to ride elsewhere regardless of how good the footing is.