No, there's no defensible rationale for the idiocy of RNG rewards for the hellish grind you have to go through to get the mats and the access to the engineers in the first place.
Not the place for this, but having just casually got myself a level 4 FSD drive and enjoying a level 3 dirty engine, I really think Engineers is still very much a problem as to how its played rather than what it is.
However, regardless of issues of the actual mechanics, the mechanics do make sense: Engineers take ships which run to a factory spec and tweak them in ways they aren't meant to be tweaked using raw or salvaged materials and components to push them beyond their spec design. That makes sense. RNG, as frustrating as it can be for some for various reasons, can fit into that ideology as we're dealing with unpredictable, experimental changes to a factory build resulting in different levels of success. Yes, there are arguments against that, but as a narrative, the gameplay does fit the immersion, even if not to everyone's desires. There are indeed some trade offs from that immersion - no one's dirty drives have blown out in the middle of space - something you'd expect from overclocking your factory ship on occasions. But that's where gameplay trumps immersion - just as no one is saying we should really RNG the transport of ships... your ship could fall foul to pirates en route... would bound to happen to some commanders. No one is arguing that as it again doesn't really balance out gameplay with immersion.
The point is, whether you like the Engineers or dislike them, and there are valid debates on that from both sides, the Engineers fit the world of Elite. RNG actually fits the world of Elite over instant-teleport. RNG is unfair, unpredictable. You don't always get the reward you want... just like life. Consession is whether that's too frustrating as a gameplay element, yes a valid topic, but it does fit Elite Dangerous. Instant Transport does not in anyway. You can't justify it as part of the immersion, and as a gameplay mechanic it damages the game significantly. Yes, Engineers could be accused a little of that. My Python loadout now outruns my Cobra with thrusters and my old Asp in FSD (no, the Asp wasn't an Explor-Asp, but 26 jump range seems pretty big for a Python). That does sort of blur ship classes, but it does have rationale within the Elite narrative and as a gameplay element (we love souped up ships!).
None outside of the same lunacy that's fuelling the threadnaught that seeks to prevent the QoL change going through here, it's the same fishbowl mentality of "IT MUST BE OUR WAY OR NO WAY"
Not seeing that - people are asking FDev to consider some of the issues that they've seen as players. Go from there. No one has said it has to be done as X really, just it shouldn't be done as Y: let's talk on the rest.
Maybe Frontier don't just cater for you lot, y'know? They have people called customers they need to reach out to, and cutting down travel grind helps them get those customers.
Yes, but so do cheat modes. And what happens when people get what they want through cheat modes? Watch how quickly they get bored of the game they're playing. When we get what we want, what we have loses value, and that's very much at the core here. This is a BIG change. A BIG reward. And in game mechanics, a BIG reward requires a trade off - and there is no trade off here. Just a simple Elite ideology breaking "cheat" to get your ship to a player without having to navigate any factor of the set-up universe. Why? because they want it. Is that really a good enough excuse? I want a Corvette? Can I instant-have one?
Offer people what they want, they'll often say yes and think about it later. Doesn't mean it's right.