If I may try give some clarification on this... and I say that you take this with a large amount of salt. On my honor though, this is based on my conversation with my local GW branch employee and not something I came up on my own.....
I think the rumors that Calderson heard about TWC being removed may be linked to the rumour that there will be no more individual codexes for the non-vanilla marines, which some might interpret as Dark Angels, Blood Angels and Space Wolves being rolled into a single "battle tome" for ALL Space Marines of all variety, codex compliant or not. Like the Black Templars before, the GW mentions that this "battle tome"" will enable a "pool" of all things that Space Marines can use.
So how is the chapter tactics addressed? Like the recent Stormcast Eternal battletomes, there will be a section to address all the different "flavors" like their chambers with their own specific Warlord traits, relics, bonuses, spells etc. This will cover most of the current chapter tactics, as well as DA, BA and SW unique flavors. If you look at the battletome, you can see for the different flavors/colors of Stormcast, which actually differentiates them a lot to the point of NOT being just stormcast with differnet colors.
On the plus side, this "MIGHT" mean that Space Marine tools which were previously not available to SW, BA and DA such as Thunderfire cannons, Centurions, Storm Ravens and Stormtalons can finally be used.
On the negative side, what does that mean for the unique units?
1) BA - Sanguinary Guard, Baal Predator, Death company, Sanguinary powers etc
2) DA - Deathwing termies, Ravenwing Black Knights, Interomancy powers etc
3) SW - Thunderwolf Cav, Stormfang/wolf, Iron Priest, rune Priests, Tempestas powers etc
According to the GW person I spoke to, those other unique flavors might be addressed in supplements to the single Space Marine battle tome. the example he gave was like the original Stormcast battletome, which then spawned the Extremis chamber, which was still Stormcast eternals but with the addition of Stormcast riding giant lizards.
Of course, the guy could be lying to me or at best, giving me his side of the story which may not have the final stamp of approval from GW. But basically, he is assuring me that while there may be no more single codexes for SW, BA and DA, our uniqueness will be preserved via supplements. Whether it is one supplement to address all the other First Founding Chapters or one supplement each is unknown. My guess it is separate supplements, to at the same time will be in line with a specific campaign or storyline relevant to them:
BA - Hive fleet invasion of Baal
SW - Curse of Wulfen and Wrath of Magnus
DA - Don't really know, either something related to a Fallen hunt or their own contribution to the 13th Black Crusade, or even the campaign at Piscinna trying to stop Ghazkul.
So to reassure Calderson, no, TWC along with other unique units of other chapters will NOT go the way of the dodo. I believe it is linked to another rumor of codexes no longer being in favor of campaign books, which is only partially true.
Of course, the implication that SW can suddenly use Centurions, Grav and the flying toaster Stormtalons also makes my stomach churn. But if the Black Templars can stomach their chapter being aborbed into the SM codex for 3 years now, I guess the rest of us unique snowflake "First Founding" chapters can do the same as well.
Again, this is my take on rumors which was then discussed with my local GW store employee. take with big plate of salt.