You may assume this is the case but there's no guarantee that diff will increase any further and even if it does there's still no guarantee that it will increase consistently for 12-24 months.
If you look at LTC diff chart from Oct 2014 to Oct 2015 when Titans and A2's came on stream, diff increased to 58,000 in Dec 2014, and then fell back to just above 40,000 for 6 months before rising slowly to 58,000 again by the end of 2015
No $ figure, but LTC / BTC dropped from 0.0115 around first of OCT 2014 (continuing a LONG fall from a late 2013 peak) to 0.00612 in mid-May 2015 when the price started to spike and hashrate started climbing again - no doubt due to the return of profitability of older gear and in cases of folks in high electric cost areas return of profitability for ANYTHING at all.
The other thing to keep in mind about the last couple years, is that for much of that timeframe there were NO SHIPMENTS AT ALL of new gear, other than small quantities of the Innosilicon "Farm Boy" units and the fairly small numbers of Alcheminers (and the MAT/etc versions) that were ever shipped. There was no way for hashrate TO increase much with almost no new gear entering the market - and NO new gear at all for almost a year prior to the first shipments of the A4.
Right now, for the first time in about 2 YEARS, we actually have more than one company making Scrypt ASIC - and Bitmain has a long history of shipping every unit of a miner design it CAN 'till it has a "newer, usually more efficient unit" available, unlike Innosilicon which tended to ship hard for a while then taper off.
At this point, I am going to be a bit shocked if difficulty doesn't top 200,000 by the end of the year (vs. the under 100,000 right now) - it might hit a slowdown for a bit when the existing A2/Alcheminer/Titan hordes get unprofitable, but even that won't stop it I am far too certain.
I suspect the current rise won't taper off much 'till the L3 gets fairly close to break-even at 3c/kwh electric, barring a major chip foundry going down again.
Makes me happy my A2s have already hit ROI - and I'm a bit happy the A4 took as long to actually get shipped after it was announced, that extra 4-6 months of high profitable mining mattered!
I do disagree that this is the end of home mining - the L3 is the FIRST "comfortable for a home miner" machine to show up in a while, with the DEBATEABLE exception of the R4. 400 watts and fairly quiet is something ANYONE can fairly easily find a spot for - unlike the other mostly rather loud and MUCH higher power consumption miners of the last 3+ years.