Sure, but competitors will not sleep forever (there are already quite a few good ASICs for altcoins) and may overtake the market of bitcoin miners, should they release a more powerful and economical miner for BTC sooner than Bitmain.
The only folks I can see managing that would be BitFury (their chip WAS in production before Bitmain's was on the current node, but Bitfury sells to BIG farms and manufacturers only so it ended up under the radar for quite a while).
Nobody else to date has managed to MATCH the S9 much less beat it - and I don't see any significant probability of that happening on the next node (very long shot if Intel, AMD, Samsung, or NVidia decided to enter the ASIC miner business, but I don't think that's likely as it's such a SMALL business compared to what any of those folks do NOW).
The reason the L3+ isn't as close to saturation as the S9 is threefold.
(1) S9 has been getting sold a LOT longer, there are a TON more of them in use.
(2) The S9 has competition (Caanan/Avalon 721/741, whatever BW.COM has been using internally, Bitfury) that is fairly CLOSE to it's efficiency that has been selling for almost a year now (bit over perhaps) and HELPING it saturate the Bitcoin market.
(3) The price rise of Litecoin during the last 4 months is over 10 to 1 - the comparable price rise of Bitcoin on a 10 to 1 ratio needed most of the last *2 years*, which has allowed a lot more time for folks to buy the hashrate up to the same ballpark as the price has gone up.