It won't be a problem to fill raids in my eyes. A DKP free raid means people are open to bring alts, mains who need items would ofc get priority. We struggle to fill raids atm because we have a very small number of raiders compared to most raiding guilds. Nite in nite out it's the same people raiding. When 5 can't make a night we struggle to fill it. We're a small guild that will happen regardless if an instance gives dkp or not. If people don't show up because they don't get DKP and they don't want to help others get loot then firstly, we should not run that instance anymore offcially as there is not the interest and secondly those people are not UNITY material in my eyes.
A fixed price DKP system is not without it's flaws no dkp system is. It is however simple to use and maintain and offers the fairest and inflation free system realisticaly avaliable to a guild of our size.
An alternative system is a relational DKP system which allows many factors to affect the rate of which people gain and lose dkp e.g decay of their points each week by a value, say 2% until they hit 0. A dkp cap which when reached their points are reduced by a percentage. Also a bidding system where players can bid 1x 2x 3x the DKP cost of an item. Meaning a player with large amount of dkp can afford to bid x3 cost of an item, almost guarenteeing them the item but taking a huge dkp hit to do so.
These dkps systems however are difficult to manage, maintain and are only worth the effort in a guild 3 or 4 times the size of ours.
In my honest opinion it is not the dkp system that has caused this problem we're faced with, it is the amount of people who have done 25 Naxx/ulduar outside of the guild, gotten the gear dkp free, come to raids as normal; no longer need and gear so their dkp just sits around doing nothing.
Alternatively elements of a relational dkps system could be applied here to manage to maximum amount of dkp people can attain, such as a decay system previously mentioned. Ultimetly the player would reach a limit on how much dkp they could keep up even if the player raided everyday, as the percentage decyaed would be greater than the amount earned.
I'm trying to offer feedback to the people who make the decisions, as opposed to saying they wont work without giving any solutions or ideas on how to improve them.