Yes, 2017 was a special milestone (50 years), but even at that, the four-day badges didn't sell out until early July. Will we see similar attendance for 2018/19? It's possible, but my gut says we'll stay flat or maybe even retract slightly.
You should be fine as long as you purchase in the first couple months (though if you want a hotel, you will want to get your badges right away, because hotels will go live not long after badges go up for sale, and you need one to reserve a hotel). In any case, I wouldn't dally too long in getting a badge; this year being a sellout, it may motivate more attendees to buy early.
Hope this helps.
This was the first year that badges sold out at all, so it is hard to know.
My own bet is sort of the opposite. I think badges will sell out faster in year 51 than year 50. I have two reasons:
1) Attendance has been 60,000 or so for each of the last 3 years. This year might have gone much higher, but for that fact that it sold out. I suspect that on site purchases would have pushed it much higher.
2) People haven't been worried about a sell out -- because it hadn't happened before and most probably didn't even think it COULD happen. I suspect this would put Fear Of Missing Out into people and they will buy earlier.