This is a thing that has been bouncing around fan circles on the internet, apparently starting off from comments made at Big Finish Day 6 about how BFP might be cutting the monthly Doctor Who Main Range and replacing it with box sets.
To be honest, I can't say I'd mind and here's why:
Over the last few years Big Finish have steadily moved away from the sort of storytelling a monthly schedule serves. When they dumped the anthology format for trilogies it was a move towards marketing the high concept of a group of stories over the individual stories themselves. They've been steadily cutting down on trilogies that are just reuniting a cast and putting them in random adventures without an overarching theme, unless a cast member is enough of a draw on their own (like Bonnie Langford's upcoming trilogy with Sylvester McCoy, which is the first time the pair have appeared together since 2006). The only Main Range stories that still really work like that are the Fifth Doctor / Nyssa / Tegan / Turlough stories and even they went high concept this year with a Return To E-Space trilogy. The trilogy is the marketable unit rather the three stories that form it.
Given that its annoying how that pesky annual anthology release means a six- or twelve-month subscription almost never matches up with where the trilogies begin or end. I'll always get one complete trilogy in a six-month subscription but bits of two others at either end of it. Box sets eliminate that problem.
They also eliminate one other very obvious problem: the monthly schedule itself. Any regular schedule breeds filler eventually and Big Finish are by no means the worst offenders but it has happened. I've often wondered whether comicbooks would be better if they went of hiatus between storylines to give the creative teams additional lead time.
There's also the fact its easier to get people excited about a new thing than a continuing thing. As atomic units even box sets in a series (the Dark Eyes boxes, for instance) are easier to drive hype for than the hundred and ninetieth instalment in a monthly series. Absence makes the heart grow fonder, as well, and I anticipate a new Dark Eyes box set or the recent Fifth Doctor Box Set more than their regular Main Range equivalent.
They could also use box sets to pilot new talent. The writing pool for Big Finish stories has shrunk in recent years with an inordinate number of Matt Fitton and John Dorney scripts (not that I mind either's work but they do write a lot of stories for just two people). If they can sell a box set with four stories in it, using one disc to trial a new writer isn't too much of a sacrifice if the other four disc are from reliable old hands.