I am not so sure. I see at least four faster and cheaper coins in the top 10 which I have used and know for sure about it.
There are still a few I have never used for transfers so far like Stellar, Eos or Tether, but I assume the last one is not a concurrence for Litecoin.
So, there are many concurrence in the top 10 for LTC, when it goes to cheap and fast transfers.
At the end of 2017 during the bubble and especially at the pick of the bubble, LTC was widely used because it was one of the cheapest coins and exactly as you said, with enough liquidity because LTC is listed on all exchanges, but this situation changed significantly in the last year. All these other coins are also already listed on every big exchange and keep the pressure on LTC.
Lately, I personally started also to choose other coins to transfer between exchanges because fees are also lower for many of them and when we add that they are additionally cheaper in transfer and faster, then this is no-brainer anymore.
Most of time, at same speed rate of transactions, if one coin (A) has lower price than another coin (B), the transaction fee at same amount of money to move with coin A will mostly be much cheaper than coin B's transaction.
Recent months, especially since the mid of 2018, blockchain technology has evolved significantly and transaction's fees become much cheaper while transaction speeds become much faster. In my perspective, it does not a problem to wait for 5 or ten minutes to get finished transactions between exchanges/ platforms and I see there are so many coins satisfy that need of crypto enthusiasts nowadays.