I am currently working on two aircraft kits, a Van's Aircraft RV-7 and RV-8. one of the kits I purchased, the RV-7, I am the third owner and the first one to even open the boxes. I am the second owner of the RV-8 project.
I don't think that an open source home-built airplane will put a dent in the number of unfinished kits. The bottom line is that it takes a lot of time and dedication to build a kit aircraft. I have several friends who have taken for or more years to complete. Many of the available kit aircraft already use the techniques Makerplane proposes using such as CNC machining of parts, match hole drilling, and many pre-fabricated parts, as well as 'quick-build' versions that fall under the 51% rule. Specialized tools will still be needed.
Kit manufacturers also leverage their buying power for raw materials.
Currently, Van's Aircraft sells the RV-12, an LSA kit aircraft for $64,000. The price includes the airframe kit, engine, prop, and instruments. Everything you need to get flying. You also get excellent support from the leading kit aircraft company there is with over 7,800 completed aircraft.
How will Makerplane compare against other established inexpensive (in aviation terms) kits? Will you legally be able to license it as a ELSA if you outsource most of the aircraft (under the current rules - no)?
Makerplane is an interesting venture I will be keeping my eye on. I hope it works out.