Whether he is replaced or not doesn't really matter. There's no definite number of merit sources to do the job. Theymos can decide to use lesser or higher number of merit sources. The emphasis is on how much merits is in circulation and not who are those circulating them.
It does matter, for what purpose the inactive merit source be still in the list if they cannot do the job, nothing. There are a lot of
merit sources application (and many more to mention) that deserves in the list. I don't know what reason theymos is holding on that inactive sources but it doesn't make any sense more particularly if they are inactive on such very long time.
Let me give you an example...
If theymos wants 1000 merits to be in circulation, using 10 merit sources can mean that each merit source has 100 merits allocation. If one of the 10 merit sources becomes inactive, it is left for theymos to replace him or increase the merits of one of the remaining 9 merit sources to 200. Or he will decide to increase the allocation of the remaining 9 by let's say 10. I hope you understand my analysis. If theymos is not accepting new merit source applications, it means he has a way of dealing with it that we don't know.
Then you will definitely get some users spend 3 to 5 merit on posts that have no value or lack of value. It would also increase the number of merit in circulation - perhap due to compatriot, brother and sister, husband, wive, relative or something like that.
Every system has its backlash, what you described above is already happening and we should just see it as a deficiency of the merit system.