Metallica already had 3 records released at that point. The NWOBHM scene was long dead by then.
The number of Metallica records released by a given year has nothing to do with when the NWOBHM movement ended. Sure, thrash was huge by '86, but that doesn't mean other styles were not also active in '86. And I said '86 was a stretch so as to include some latecomers; no one's trying to carve that date onto the NWOBHM headstone.
If someone wants to copy/paste/move the stuff below into the other thread Avenger mentioned, that's fine.
If someone wants to try to select a NWOBHM end date, without being rather subjective, here are some ideas:
1- define a baseline for how many new bands must enter the scene by releasing a vinyl item in a given year for the scene to still be active, then see which year in the 80s saw NWOBHM new bands' 1st releases fall below that given line.
Example: If a scene needs 12 new bands per year to break through with their first vinyl release (avg 1/month), then find the year when less than 12 NWOBHM bands released their first vinyl offering.
Rationale: once you stop seeing enough new bands enter the scene, it's a good indication that new bands are instead playing different styles and your old scene is no longer growing (though older bands may keep it afloat for awhile, so #2 below may work better).
2- define a baseline for how many vinyl releases a scene should produce each year by all bands involved, then see which year missed the mark.
Example: If a scene should produce at least 24 new vinyl offerings a year to be considered viable (avg = 2/month), then find the year in the 80s when there were fewer than 24 NWOBHM vinyl releases.
Rationale: A dearth of releases indicates new and old bands aren't very active and/or they are changing direction musically (e.g., some glam bands quit in the early 90s following the grunge explosion of 91-92, while others tried reinventing themselves as grunge or some other style, like SHOTGUN MESSIAH did with their 'Violent New Breed' album in '93).
Defining the baseline could be tricky, though you could start by setting the baseline as some percentage of the number of NWOBHM vinyl releases put out in the year that saw the most NWOBHM vinyl released.
Example: We decide the scene will be considered 'over' once the number of vinyl releases falls below 20% the amount released in the peak year. Let's imagine that 1981 saw the most NWOBHM vinyl releases with 100. We would then identify which year after 1981 saw fewer than 20 NWOBHM vinyl releases.
In theory, the number of releases per year should follow a bell-shaped curve.
To be clear, I ain't gonna do any of this! Or at least not until the next time I'm confined to bed rest for a week following surgery!
chosen as the unit of measurement b/c CDs weren't a thing through much of the 80s, demo tapes can be churned out by almost any basement act, and bands who played live without releasing a record would be hard to get a tally for. Counting vinyl releases could accurately be done, though likely with some disagreements over which releases qualify as NWOBHM).