I get ya, you are good folk and want honesty and all that jazz. I'm the same and I always try to do the right thing, but let's be real for a second, and please don't take this the wrong way, just bear with me a little.
Flash sales are the ONLY thing worth spending dp on, everything else feels like a rip-off. This is literally the one system no one can abuse, you need the dp, you need to lvl up, for that you need ap, the time, etc etc etc. There is virtually zero reasons to not have those sales with the same frequency. It's even good in a business point of view, a lot of players put cash to buy those sales everytime they appear. More frenquency means more cash into their pockets, in the end.
Now, let's look at some other stuff that we actually have reasons to worry about.
Starting with vip system = paying players, which should be on top of their list, right? Wrong. Look, you pay, here are some glowsticks. Have fun wih those because there isn't much else going on with vip system! Oh, but some are getting both normal and vip rewards, some are not? Is that even fair? Will the devs give compensation for those who didn't get both? Will they take away every single extra item of those who got them? Nope, all remains the same. If you are lucky to get both, hurray, double rewards! If not, sucks to be you.
How about when some people got birthday rewards without being on the date they set up as a birthday? It was posted a while back. Did everyone get those rewards? No, just a few because of a minor bug. Did they give some sort of compensation? Also no, but those who got it kept their items. Fair? Depends on who you ask.
Eh, wait. There was also that funny time when they screwed the first two UR+ colors. Lots of people where there, farming and spending their precious ap and dp to get skulls and other materials, and then one day poof, they changed the colors required to what it was supposed to be. Cool that they have fixed it, but everyone who was farming the wrong color because of devs mistake - some even skipping events just to level up those cards - had to start from scratch. What about all that ap and time taken away from players? Nah, not important I guess. Here is 10k grimm as compensation or something.
Ah, but new lessons finally came out! Yay, thanks Solmare you rock! But wait a minute. Yup, they took out half of the hard farming stages and put it into the new hard lessons, which is fine... except for everyone that is trying to bloom their cards to DO those new lessons in the first place. If you can't do the new hard stages yet then sorry, ya just gonna have to deal with farming skulls with only 3 places avaliable instead of the five or six that you could previously, before the update. Don't like it? You are in luck, because they put skulls in akuzon, so you can buy it... with real money!
Why is this crazy person writing a freaking bible on my reddit post? Well, what I'm trying to say is there's a lot more going on that players should pay attention other than flash sale frequency. In the end, if there is something wrong with said system they will end up fixing it, but right now it's just not as important as some of the other stuff they should be looking at.