I'll bite. Aside from a void in their lineup, what void do you think it's filling where there's not another plane that does the same thing?
It is clearly obvious at this point that it doesn't look like the RV-15 will fit YOUR needs but that doesn't mean it wont fill the needs of many others.
How about if we flip the discussion and ask you to list all of the other kits that already directly complete with the mission profile that the RV-15 is designed too.
A few of the key ones for the 15 are -
Very good cruise speed to landing speed ratio. 140 kts (to get you to your adventure site relatively quickly) to 400 ft take-off / landing distances.
Equal comfort seating for two, with no difficult gymnastics required to get into the front and/or back seat (one thing we have learned in 50 years is that this is a real issue as the pilot population average age keeps creeping up), and the fact that for a lot of the pilot population, having their passenger seated beside them is a huge selling point (compare the # of RV-6's and 7's sold compared to the # of RV-4's and 8's)
Huge baggage area
I have already calculated that I will be able to carry two mountain bikes (only disassembly will be removal of the front wheels), my tandem seat inflatable kayak and associated gear, all camping gear needed for two people including an inflatable air bed (I am getting older too), a good sized cooler with all food needed for more than just a weekend adventure, and all of the other misc things that I might want. We think that capability work for most peoples needs, what every they may be.
A landing gear design that gives the same level of energy absorption / suspension as other aircraft have, but without using the huge / soft (and very expensive) tires that many other aircraft require, to get the same performance.
Very quick and easy to build once the expanded detail construction manual has been fully developed and released.
These are some of the more significant design details that we think make the RV-15 stand out in the market, but there are many other smaller ones as well.
Two things that it does miss is that it is not tandem seating and it is not a full on 4 place airplane.
That is for a very specific reason... it influences many of the other design benefits, such as large baggage area, etc.
Extensive research showed that the majority of people flying the same mission with 4 place aircraft (C-170, C-180, etc.) do so with the rear seat removed, so they are operating them as a two place anyway. Doing so is just compromising the overall performance because you are flying an overall larger airplane when you otherwise wouldn't have too.
The side by side detail I have already commented on.
We have learned a few things after 50 years in this business. A significant lesson learned is that we can't please everyone. But that doesn't mean what we do in a design is wrong, or we probably wouldn't have lasted for this long.