It really does make sense, plot-wise. We also have to consider how large the cast had gotten by Season 4; it makes sense to cut the characters down to a more manageable number, so we're not split between four storylines like last time.
*cries* I liked Argyle. Hopefully he'll still get mentioned tho and he and Jonathan are stated to stay in contact (Argyle did say Jonathan was his only friend so its likely).