You need to supply both types of kits. Not both types for each project but surface mount when and where it is necessary.
Just remember this: Kit sales are less than one-hundredth of what they were 10 years ago.
I have over 350 kits and all kits have been designed by me. Sales are 1% of what they were some years ago and you need to design a kit that is completely different to anything on the market to get any sales at all.
The only problem with surface mount kits is you have to buy the components on reels of 1,000 to 3,000. I can help you with some surface mount components as you are not going to use 3,000 surface mount diode bridges or regulators. You also need to get panels of printed circuit boards to keep costs down. This is where you put as many boards as possible on a large panel.
Boards should cost you no more than 10 cents per square cm for single sided boards with solder mask, tinning and legend. That's why you need lots of projects to keep costs down.