Double-clutching is what commercial truck drivers do because their transmissions don’t have synchronizers, and if you do it the way they do it, you will definitely grind gears, because they only depress the clutch halfway when they do it. Truckers only depress the clutch all the way when they start out from a stop. You’d also waste a lot of gas as you rev the engine as you downshift to manually synchronize the gears.
If you do it right, you will save wear and tear on the synchros. This will only matter if you intend to keep the car for over 100,000 miles - else they won’t wear fast enough either way for you to care.
You won’t wear the clutch out faster by double clutching if your technique is good.
When I started driving, all manual shift cars had 3 speed transmissions except Model T Fords and automatics were much less than 10% of the market. All these 3 speed transmissions had syncros on second and third gear but not first, If you wanted to engage first gear while moving, you HAD to double clutch. When I started driving tractor trailer many of the tractors had no syncronized gears and many had 15 or 16 speed trannies with 2 shift levers. They came with 5x2, 5x3 and 4x4 varieties and were actually two transmissions one behind the other The early diesel engines only had a 300 rpm operating range, from 1800 to 2100 rpm. You got to be very good at double clutching or alternatively slipping the lever into neutral as you were letting off the gas and unloading the gear, then blipping the throttle and slipping it into the right gear. I doubt if there has been an on road tractor since the late 70s sold with non syncro gears.
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In my case, I’m going on 280,000 miles on the original manual transmission, including the synchros. I think the synchros on a well-designed well-maintained car should be one of the last things to break.
Also, I have to wonder, how will double-clutching help synchronize the speed of the gears on the car if you’re letting the engine idle? How would double clutching help the synchros last longer? I don’t see how pressing the clutch twice synchronizes the gears without throttling the engine as you downshift.
At best, I see this as OCD behavior that isn’t going to prolong the life of any part of the car.
I learned to drive a truck in 2006, and I spent about a year working as a truck driver. There were a few trucks I saw on the road that had automatic transmissions, but every Class A manual transmission truck I saw or drove still had non-synchro gears, and that included both during and after truck driving school. I’ve seen some Class B (shorter, with permanent trailers) that had synchros, but no over-the-road tractors.
Non-synchro gears are still a thing in the trucking industry.
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Well, for that matter, it is HEEL and toe, not “heal” and toe.
Double clutching will wear out your knee 
However, drive the way that makes you the happiest.
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Synchronizers were designed to make double clutching unnecessary.
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I’ve been driving manual trans cars since 1985. I’ve never heard anyone talk about double clutching or rev matching until the last few years. There is no reason to do this stuff unless you want to wear out your clutch twice as fast.
If you’re letting the engine idle, then you’re not double clutching properly. 
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Whitey, I was going to argue with you but I researched the subject and you are more right than I am, In 1972 the company I was working for got 112 new Kenworths with the then new 350 Cummings, The safety department told us to not shift the 13 speed Road Ranger transmission at over 1800 rpm in the low range because the syncronizers in the transmission were not strong enough to take the torque at full rpm in the lower gears. I just assumed that all the subsequent trucks had syncro transmissions. I was in trucking from 1955 to 1995, guess shifting is just something you don’t think about after the first few years. I doubt there were any engine/transmission combos i didn’t encounter driving for various freight companies. Most of them did not have any assigned equipment and you drove a different tractor almost every trip. The worst were the old duplex, triplex and 4x4 transmissions with two sticks. If you missed a shift going uphill with a load, you just had to pull over , stop and put in 1st low and start over. Thank god for the turbocharges that gave diesels a much wider operating range. I pulled long doubles on the NY Thruway and Mass Pike with a Mack with only 5 speeds grossing legally 143,000 pounds.