Eventually the difficulty curve we have seen on scrypt and 256 will level off. Scrypt is a different beast with asics because there are so many coins to absorb the hashrate. Scrypts earnings dropped faster than bitcoins ever has, but its leveling off faster as well.
I beg to differ. Number of coins doesn't matter much because LTC is ~80% of all Scrypt hashrate. And at the current rate of ~900 GH/s a couple of thousand of these Titans or Vaultbreakers or whatever can easily DOUBLE the difficulty, if these miners are actually delivered and up to spec. Even Bitmain with it's L1 can slaughter Scrypt profitability. I don't know why it has leveled off recently, maybe because Zeus hardware killed the profits for itself and of course for GPUs, maybe because GAW stopped selling hardware, but I wouldn't expect this to last. More efficient ASICs will come eventually.
You are free to differ in opinion, but your wrong. I presume you know how the multipools work? They switch coins based on btc value and difficulty, most of the time you will find that they are not mining ltc, they are mining ftc or doge, etc. If what you say is true, then the average payouts would still be declining, they haven't. Look at poolpicker.eu, you will see the 30day average earnings are just shy of 0.0004/mhs, and thats average, so that means there are plenty of days where the earnings were higher. For 30+ days the earnings have leveled off. LTC might indeed have 80% of the hashrate at any given time, but plenty of other coins have dedicated communities who would buy asics to only mine that coin and this doesn't include the multipools who jump from coin to coin 10s or 100s of times a day. Pools like coinking split there hash between smaller coins so as not to kill them.