But the fact of the matter is that BTC is in the enviable situation that there is no one bigger that it using the same mining hardware. You are not in that situation, and nor are we. [EDIT: And unless we change our PoW, we will never be in that enviable situation unless/until there are custom Cryptonight miners. And unless you change your PoW you will never be in that enviable situation unless LTC fails - or you grow bigger than it.]
Perhaps both will happen
I don't know anything about Bitmark, so I don't have an opinion on how likely it is to eclipse LTC.
As for whether Monero will ever have custom miners? I think probably, but I haven't thought about this deeply. I doubt we will have Cryptonight ASICs any time soon in an analagous sense to SHA-256 ASICs. But then, custom designed machines that miner more efficently than a PC or graphics card? Very likely. And in the long term, will such devices use ASICs to assist the process? Probably.
But my belief is that the factor by which the custom hardware improves the hash per dollar over PCs and GPUs will be way lower with Cryptonight than with BTC. And therefore I think out network will be less secure that it could have been. My guess is Scrypt has the same problem in being worse than SHA-256 in this regard, but that Scrypt is better than Cryptonight. (Except, as I said, I wouldn't use scrypt, because there is already a big established scrypt coin which will be difficult to displace)