In No way am I dismissing the validity of your feelings, yet of the 30 people I have played 5e D&D with, only 3 people, including myself own SCAG.
I bought SCAG for the same reasons I bought Frostburn in 3e.....I have "completionist collector tendencies" and I saw that Booming Blade and Green Flame Blade are clear and present Power Gamer fodder.
I predict Tasha's is going to sell better then SCAG. Many players, do not follow Errata or Sage Advice....so why wouldn't you update two spells that any reasonable person would conclude, were sloppily written, in a product that is going to be widely purchased?
I hate errata PDF dumps, so updating The spells in Tasha's, is more preferable to me.
As for the alterations...Sorcadins can still Quicken Booming Blade. Twin Spell and Booming Blade Action economy had some wonky interactions....so sacrificing a Twinned Booming Blade for clarity and balance is something I am fine with.
Eliminating Booming Blade from working with the Warcaster Feat, I am less sanguine about. Ultimately, I think the change is for the best, it forces those players that tend to use the same build, or the best option over and over again to change.
This keeps the game fresh, which keeps people engaged.