It's a totally relevant! Rather than test LN and its base code SegWit, (bitcoin's DAO), in the real world befor deploying it on a $10B network
LN's basecode is not Segwit. I'm surprised that you have been fed false information. Segwit is not Bitcoin's DAO and is not even as complicated as some people say that it is. Most of the codebase is just automated testing. LN could be created (AFAIK) without Segwit but it would be much harder.
why shouldn't SegWit be deployed and tested in the real world on an altcoin before being implemented in Bitcoin?
Nobody is preventing altcoins from implementing Segwit. I'm 99% of sure that if the Core developers decided to wait for this 'testing period' that the 'shill campaign' would complain everywhere that Core is stalling progress, fee market panic and other rubbish.
I know, sure SegWit comes bundled with a bunch of other changes among them the implementation of bitcoin's a scripting language on which LN will be dependant. No need to trust me but it's way more complicated that you alluding too.
I don't see any shilling campaigns maybe you can enlighten us? I do see a bunch of digital influences paid by huge corporate sponsors to create FUD around the dangers of bigger blocks.
I'm glad to see there is no resistance to altcoins implementing Segwit, I suggest Core got on that and do some real world testing before messing with Bitcoin implementation, what do you think about deploying it on the Litecoin network? it shares a lot of fundamentals with bitcoin.