I think the issue people are having is that FW is being neither consistent nor upfront.
It'd be one thing if the issue is that FW doesn't have the space or resources to dedicate transfer sheets, heads, torsos, shoulders, Rhino, Deimos Rhino, and Land Raider sets for all 18 legions, so they've decided to let the resin sell out as it sells out and stick to transfers. But they haven't communicated that, so you end up with random differences in what legion has what sets, and there's at least two transfer sheets (EC and Blackshields) that never came back - which implies that's not the strategy.
For some reason, it feels like FW is stuck in a mentality that they cannot just openly and directly admit in a WHC post or email to all that, hey, we can't print on demand as needed, and we can't stock everything we ever made, so decisions and cuts need to be made, and we are making them based on whatever - what doesn't sell well, what customers demand, what seems redundant, etc. Or "hey, we can't keep everything in stock simultaneously at all times, so here's the core all the time range and the rest will get once a year Made to Order releases for you to get (ie June is Iron Warriors and Emperor's Children, so all their upgrades and sheets will be available for pre-order on May 30th and additional stock will stay in the store from June 1st until they sell out)").
Or whatever. The frustration is that without any explicit direction or explanation from FW, people are left reading the tea leaves, and the tea leaves don't make sense. It makes sense for FW to stop producing non-Deimos Rhino doors ("this was a bit redundant and frees up 18 slots for more unique stuff"). It makes sense for FW to stop all shoulder pads and doors ("this frees up 72 slots and lets us consolidate all legion markings to just 18 transfer sheets"). It makes sense for FW to stop the basic legion head and shoulder pad upgrades ("this frees up 36 slots and tbh a lot of these just weren't selling"). It makes sense for FW to drop the upgrades for Ultras, BA, DA, Scars, Iron Warriors, Alpha Legion, Night Lords, and Imperial Fists ("we know it may not be quite the same, but GW makes plastics or transfers for these for 40k and it honestly lets us free up those inventory slots for more HH specific things").
What doesn't make sense is taking a 5 year old's self-haircut to the range, with seemingly random gaps and discontinued items across the items, with no guarantee or explanation of what to expect. Can I focus on saving for a big tank in the near future or is there zero guarantee that those Iron Warrior transfers are staying in stock past the next "out of stock" situation they encounter, whenever that may be? Or do I buy the transfers now but risk the tank going out of stock and never coming back? (which I'm currently dealing with with the Gorgon transport, when I failed to pre-order it in time before it went out of stock during the pre-order).
And right or wrong, none of this is helped by the 180 that GW proper has done in terms of social media and communications. Yes, they're still corporate. Yes, plenty of stuff in the main ranges goes out of stock without notice, or sells out on pre-orders, or takes forever at times to restock (I got lucky and stumbled across all six copies of the plastic Zone Mortalis tiles I need for my 6x4 table when they were released, as they've been out of stock since). Yes, 8th ed has its own rules issues.
BUT...GW has been winning the hearts and minds of its gamers and customers. Nobody doubts the support of either 40k as a whole or any of its armies. It's generally the truly "fringe" or outdated models that get culled - when a relatively core kit goes unavailable the question is usually not if it's been dropped but if it's actually being replaced with something new.
This has made FW look a lot like the GW of old, where it feels like things are being done either for arbitrary or coldly bottom line based reasons, and nobody's hearing a straight answer, let alone a promise or guarantee, of what to further expect. Which in turn makes it feel like the whole process is either a symptom of FW just not caring, or if this is a cynical sales ploy to scare gamers into panic-buying as much as they can under the vague threat that it won't be around in two years or six months or next week.
And...to be blunt, right or wrong, people trusted Alan Bligh. I don't get the sense that people trust FW. That may not be fair, and there's more important things in life than Horus Heresy models, but Alan's passing I think has triggered a certain sense that HH no longer has its protector/evangelist/guiding light, which in turn makes it easy to wonder if anyone "important" within FW/GW actually cares about the range, or cares enough/has enough clout to keep it viable or protect it when the latest expansion to Aeronautic Imperialis or Sisters of Battle or the return of WHFB starts making demands on warehouse space or inventory codes or production schedules.