MBE550wow

Hi everyone,

Need some input here. 3 weeks ago my passenger low beam went out, (xenon) so I ordered a replacement bulb from an online MB dealer. I thought it simple enough so I changed it myself. Fast forward to this past week, the light went out again. I had a mechanic check it out, and he states I didnt put the back on correctly, so the headlamp assembly corroded, and now I have to replace the ENTIRE headlamp assembly, $850+ for a new original part..... attached is a pic of the corroded part he gave me....

My question is, I find it hard to understand why we would need to change the glass, etc...is anyone familiar with this problem? Cant this part of the lamp unit be changed? Any input would be appreciated, I am now scouring the internet for a used part to save some money.....
part that is corroded