If it simply boils down to a matter of cost (which it doesn't since there is a range of HT oils out there and my research has shown many to be cheaper that 5 gallons of new ATF or motor oil), then why not apply the logic to any part of the equation, why be so selective with the logic? Why not use water, it is free? Is it simply because the warping and cracking is plain to see, too slow a quench is equally problematic we just can't readily see the issues. Why not use free mystery steel? Why not eliminate the heat treat altogether, grinding the blades out of files while continually dipping? Handle material is expensive, why not chunks of 2x4 for handles?
Think about it, the same guy who may wish to use free quenchant or free scrap steel, would have a real problem with handling out the knife with pressure treated pine even though it were free. He would probably attempt a finish that would eliminate forging scale and many of the scratches, why? Probably out of pride, part of him wants to make a good knife he can present to others. Yet the MOST important part of it being a real knife is subject to the corner cutting and compromises, because the public cannot readily see it. I guess it is matter of personal taste, but I would rather put Woolmanized pine on my handle than have a blade not cut as well is it could.
One does not have to spend a fortune to get started in making knives, other than some supply orders on my credit card in all of my years making knives I have NEVER went into debt to do this, even with all my fancy gadgets that most would never need to do it. But after all these years I am better situated to rough it and get away with improvised tools and materials, because I have the experience to understand the pitfalls, work around them and make it work. The guy starting our really owes it to himself not to handcuff himself with unnecessary obstacles, it will really mess with the learning curve and will almost invariably lead to the spend an extra dollar just to save a dime scenario. Why not hit the market with a $500 knife that can cut like a $1000 dollar knife made by the established guys? Seems that would be a much better marketing strategy than having to spend years proving that those "good enough" knives were just to get you started. Why not be the new guy who's knives perform like a pro's?
I am more than happy to allow those who have been enjoying the results of various glops and sludges to continue on in their contentment, but I have a privilege (responsibility) equal to theirs to give my perspective when a new guy needs input on how to do things. Viscous goops and used motor oils will make blades that will bend to 90 degrees very nicely in other shops, in my shop they do this:
With selective heat treating one can also get 90 degrees out of a blade with none of that fine pearlite in it, if that is their bag, but it will also perform in other areas equally as well instead of sacrificing and compromising in order to make the bend. Or worse, settling for lesser performance until you are famous, because with the competition these days that mindset will never get you there. As a collector would you be eager to support a new maker who is willing to settle or compromise on performance? And as a hobbiest why not have the first knife you make be the best it can be? If it is the only one you ever make why not have your grandchildren amazed at how well that heirloom cuts after all those years? As the old adage goes- "If it is worth doing..."