I read somewhere that 2017 new vinyl sales in the UK will pass 4 million - the highest in >30 years iirc.
Now the largest-selling album format. Long may it last 
In meta terms, vinyl sales are slowing down.
It will remain a niche product. Interest will plateau and sales will drop. But it should remain mostly viable for those companies currently involved.
The who what and when of vinyl sales is interesting.
“Vinyl sales set to soar past 1 million in December … Vinyl has experienced a boom in the past decade with sales up by 1,472%. … reports that almost a quarter of those who expressed an intention to purchase a record turntable in the run up to Christmas are aged under 35 and almost half (46%) are female.” (Musicweek)
How many stick with the format to the point of progressing into collecting or upgrading equipment will be a fraction of this number. That said record fair attendance is up which is great to see.