Firstly,
Your friend Scotty sounds like a mechanic who has finally worked out he can make more money from presenting You-Tube videos than he can fixing cars.
Most of what he said was total trash apart from the line " no fancy plugs will magically increase Hp or give better fuel economy".
I ride motorcycles from the early 1900's and have been doing so for 50 years.
The reason they were made in 2 pieces is because the fuels were poor the clearences in engines were huge and most ran total loss lubrication systems so the plugs carbon fouled a lot .
Nothing to do with the "softness of Cu" and I have some spark plugs that would have done way over 500,000 miles because you just keep cleaning them then adjusting the gap till there is no electrode left.
And on these plugs the electrode is usually Be - Cu not 99.99 Cu .
For the same reason as mowers not have forged in ends on tie rods rather than a replaceable one , the 2 piece plug got replaced with the one piece plug.
And the developement of stable high temperature alloys for heads facilitated using a smaller diameter spark plug while the limiting of space within the heads of OHV engines where exhaist valves were huge before WWII forced the developement of high hot strength valve steels pushed the requirement for smaller plugs as well.
Nothing to do with the strength of the copper at all .
And the story is the same for Pt & Ir .
The rate at which the electrode errodes has nothing to do with the hardness of the metal used . It is all about the crystal structure & orientation as by the way is hardness which is a function of the number & orientation of the slip planes within the grains .
Pt plugs came from WWII as well to prevent carbon fouling and to resist melting when engines were over reved at 30,000 feet when under lean burn conditions & running extremely hot, then had to be able to run in rich burning conditions found at ground level , particularly at take off .
Then he flapped on about hard to reach plugs, well there is a special bevel drive spark plug made for just those jobs.
If he was not so cheap he would have . I have one we bought it to do the B bank on the RR 6.75L V 8 engines fitted to the late Clouds, Shadaows, Spirits, Spurs & original Azure .
As for one brand over another that is and always will be trash.
during my undergraduate years I worked at the Champion spark plug factory at Alexandria NSW.
The same machine made the plugs for Champion, NGK & Lodge as well as all of the store / marque plugs like Motocraft & Autolite .
Now the real "problem" with Champions is they have a coarse heat range grading.
If you check them against NGK or Bosh you will see that the champion often spans up to 5 heat ranges for the other plugs and in many cases it gets used right on the limit of it's range so any other engine problems will cause the plug to fail.
As for fitting Pt or Ir plugs in a MOWER engine, nothing wrong with it if they are within the correct heat range other than being a waste of money.
Same as running on 100 octane fuel, won't make any difference other than costing more money .
And synthetic oils , same story .
The other problem with standard plugs is there is next to no profit in making them so the spark plug companies push the more expensive plugs endlessly and all of this marketing does sink into the back of peoples brains.
A std plug would be lucky to make the plug factory 1¢ of profit where as exotic plugs would bring in $1 to $ 3 of profit so a no brainer which plug the factory wants people to use .
Now if this was the Ferrai , Lotus . F1 or other forum dealing with engines that are being pushed right to the edge of their limits then your arguements would have some merit.
However this is a mower forum
Mowers use the cheapest lowest capacity least stressed engines it is possible to make.
Quality is not a design trait in any of them now days it is all about supplying them to the mower companies for 0.5¢ less than their competitors .
As such they will always have standard plugs fitted and there is a better than average chance that the plugs are supplied for free in the hope that the 1 plug fitted to the new engine will result in 10 to 20 bought during the engines service life which would go a long way to explaining the very short plug replacements recommended in most engine manuals.
FWIW I have around 150 commercially used mowers in my service run and I would be lucky to replace the plugs in them any earlier than 500 hours and some have been running the same plugs for the 8 years I have been running the business .
Hand helds are a different story but some of them are running near the limit of the plugs and at 10,000 + rpm slight differences in the plug does make a difference.
At 4000 rpm you could just about use a rusty nail shoved through a cork .