This is the one.
Imo its an easy solution. Make the models. Like for the Ymgarl, all they needed was an upgrade sprue, and bundle it witha normal box of Genestealers. Given that I don't see much anyone making x in bike characters, I can't see why that has to go. Literally the way most every one made them except lsvish scratch builds was buy the single bike box from GW, buy a character, hacksaw his probably metal legs off. On the other hand, to build things from the box you often cannot even build the base unit. Take Techmarines. Now that they stopped the two with a single servo arm you must take the harness upgrade if you don't convert them down. Personally, spare Servitor arms from the gun servitors of one unit onto a modifed backpack, use the tank gunner mechanicus shoulder pad, maybe mk3 iron legs if you an find them. So clearly, its not consistent, but then consitancy has been difficult for GW for years.
And what it hasn't stopped is other companies making heat rays, fusion beamers, ion guns, thunder cannons, flamer pistols and combination versions there of, even combination heavy flamer/heavy rocket machine gun which is undoubtedly a DW infernis heavy bolter. Or infernus. You know what I mean. Or jump packs.
All these conversion parts exist and are now even safer from gw's legal so long as they stay clear of trademark names and lookig way too similar. Even if some get quite close.