If they knew it was going to suck, why bother making it? Why is it considered a spinoff? Did they just not want to make a new entry in the main series because it would take too much effort, time, and resources? Were they going to replace the main series with it? I hope not. They better rectify this.
They are not replacing the main series. They're just trying to
replace Shining series with that. Here's some brief explanation I read from a Neogaf member :
Back then most of their strategic choices for their japanese IP were dictated by the japanese market, and Valkyria Chronicles didn't sell well at all, for all purpose it was mostly using the core gameplay mechanics used by Sakura Taisen since the third entry on Dreamcast, with the latest entry on PS2 being a huge disappointment in sales (at its peak the IP was a 500k seller, third entry was >300k, Sakura Taisen V is <200k) they probably decided to move forward with a new IP for a fresh start on PS3 instead.
The development costs for Valkyria were without a doubt notably higher and guess what?
The game had the exact same sales than Sakura Taisen V, which is the last entry this series ever got, on a financial basis Sega Japan probably took a hit there
In order to salvage the IP, Japan then got a budget version released one year later that started decently (because of good word of mouth likely) but really had a spike when the anime adaptation started on TV, anime adaptation which changed a few details in the story and clearly prefigured what was to come for the IP later on (a character design trying to appeal moe anime fans, some dumb love triangle with Welking Alicia Faldio etc.)
In the end it made the game go from 150k to 250k, that's better but still nowhere near enough to make it a relevant IP on the home console market considering the costs involved, PSP was also booming at that time in Japan so in that context it was not so stupid to make the jump which they did and... while it didn't sell wonder Valkyria Chronicles 2 ended up at 150k, yep the exact same sales the first one got on PS3 originally but for a lower cost.
It was still not enough, but with the investments done so far they decided to do a third entry one year later that went back to its roots, the game was made from a lot of rehash and done by an external team Media Vision (which kinda become the team developing RPG for Sega afterwards with two Shining and Valkyria Revolution) it was fine for what it was and a good product and sold... 150k once again.
They tried a cheaper extended rerelease 10 months later which didn't do much (20k maybe ?) unlike the original VC.
What happened for Valkyria Revolution is that after two Shining games made by Mediavision (the IP they decided to go with the next few years instead of Valkyria), the latest being an action RPG, it was probably decided that when pitching their next project late 2014, instead of doing a new Shining it was maybe better to go with the Valkyria IP instead, domestic sales would probably be rather similar (Shining are 100-150k seller) but the worldwide appeal of Valkyria will make it worthwhile to localize the game for western markets (the two Shining aforementionned never got there, although being on PSP and PS3 probably didn't help), at that time VC wasn't/was barely released on Steam, it was also a side project Sega Japan had nothing to do with.
As for western sales, it's still worth mentioning that the original VC didn't sold gangbusters at launch either, it was rather quickly discounted (I remember having a discussion on Gaf on that topic one or two years ago, I'll try to dig up what it was about in details later on), as for the PC release while its raw sales number is incredible (it's around 1M) it should never be forgotten that the game was released at the 20$ pricepoint and most of its sales happened when the game was heavily discounted (5$)
Overall 1M sales for an average 10$ makes the same money than 200k sales at 50$.
And let's not even talk about the "first to be there" syndrome where every japanese IP releasing games on Steam had a dramatic drop in sales for its second entry.
Valkyria Chronicles 4 may/will/is happen(ing), but if it does it was always a project that had nothing to do with Valkyria Revolution to begin with, which is also what Sega always said when they were asked the question.