This thread is the pinnacle of misguided indignation.
You don't actually lose anything. You keep your G5 mod unfortunately and if you convert to the new system, it becomes a mod as powerful as what G5 now, with scope for further powah increase if you didn't have a god roll.
So you are literally complaining about the fact your mod has the number "4" on it rather than the number "5".
Want to ask a better question? Ask why existing G5 mods aren't forcibly converted to G5 in the new system.
You are in error. New Grade 4 engineering mods do NOT match old Grade 5 engineering mods for power or capability, unless you had a terrible roll and had no materials to re-roll, nor time to get more.
I know FDev has claimed new grade 5 engineering mods to be ~5% more powerful.
But this has already been proven wrong by several YouTubers, who did the conversion, ground up to max level 5 again and found that at least several engineering mods are pretty much identical or less than 0.2% better. Not 5%.
So there MAY BE a number of new G5 mods that are 5% better, like FSD - an example where FD mentioned specific numbers. But the rest of the mods not talked about, don't take the hype for granted...
To your last point: If G5 mods were forcibly converted to new G5 mods, I wouldn't complain. I just don't want to loose my existing G5 mods, many of which were extremely costly in grinding time, due to requiring many many rolls.
I'm not blind to the one benefit of the new system - you can buy an experimental effect for your G5 engineering mod after the fact. And you can switch it without having to re-do your entire mod - with the RNG stat generator...
But overall, it'll be 5 times the grind, the materials traders are a rip-off and the theft of the 5th grade is inexcusable.