Why buy a kit to repair your faulty 888LM when Liftmaster will send you a properly revised one for free?
This is a reasonable question. The key to my perspective hinges on your use of the word "properly".
My response is that, with a 25 year career in tech I feel like I can tell when a solution represents a design compromise. The 888 is tasked with communicating and powering on a single pair of wires and that to me is it's achilles heel. The documented failure model ends with your garage door open which can be catastrophic in an urban environment and isn't typically what anyone wants.
A design change to address this could be any of (battery backup, option for 4 conductor wire, a/c adaptor, two piece module with one attached directly to the 8500 and all communication between the two done wirelessly, etc). At least some of those options retain backwards compatibility.
If they have indeed made circuitry changes to eliminate all chances of the current failure mode, great! Without doing a complete teardown and analysis (which I'm not qualified to do anyway) I can't say if they have or haven't. My gut says they didn't make significant design changes as that would be costly. I suspect the purple dot models either use different capacitors or have a modified charging cycle frequency to improve expected capacitor life. The problem is, no amount of load testing can prove that the revised 888's will not fail with the same lifespan and there is nonzero risk that the changes could introduce new trouble or result in an even shorter lifespan. My inquiries to liftmaster have gone unanswered. I'm sure they are reluctant to explain anything for liability reasons and they have no responsibility to do so, I'm just an n of 1.
Instead of focusing on the question you ask, I took a step back and asked myself, "what do I gain with the 888 which I don't have with the 78?". The motion sensor never worked for me, it frequently left the lights on all night. The internet connectivity helps detect user error but does not detect system failure. The answer was, "not much." For the most part, the significant difference to me is the look and feel of the form factor and the ability to open the door with my phone when I arrive home on my bike without having to carry an extra remote.
Meanwhile, the 78 has a long history of reliable service since it's decades old. The circuitry is basic as a switch should be, it follows the KISS principle. It's instantly available after a power outage and does not beep, something I find rather annoying about the 888. The light button on the 78 seems to work a little better than the one on the 888.
The 850 has significantly improved range which makes life easier for me as it avoids having to get out of the car to close the door once in a while.
Cost-wise, giving Liftmaster an extra ~$100 to gain confidence that my garage door won't open while I'm gone is reasonable to me. I do like most/all of their other products, I just don't trust the 888.
For me, the change was a net improvement though a minor aesthetic downgrade.