Getting new labs should be pretty expensive, at minimum make them like 200 million at start, and consecutively more expensive per new lab purchase, these things are the meat and bones of this game, having the ability to perform 3 designs/revisions compared to some other player company, is a massive advantage. Sure it's another massive financial investment to have to support another design and revision per turn, but with more designs means you get more income, because you will get more tools to market with.
Secondly, what's stopping me from putting all of my designs always to the marketing phase to both sides, if there effective enough for marketing? Is it possible that the competing nations during the war are using the same weapons/technology and get pissed off, once they realize their arm's employer is supplying both sides of the war. If the countries find and catch on to this, will this make it more difficult to market your new designs to them?
Conclusively, this is about the country's economy. Does each country or nation have a limit to the amount they will spend? If for example you keep giving them widely-different toys that perform different roles, wouldn't you always have a demand. Would this mean that never trying to make a "mark 2.0" or improved design ever a good idea because it obsoletes your previous design, which means you will lose income. The only scenario I see, is if you made a tank design and sold it to the country, another competitor provided a better tank to that country, you have the possibility of getting a better tank, to maybe attack the competing company.
The question I'm trying to ask here, is their a limit to the amount of "budgeting" each country has on spending money on these contracts for tech. Also are these designs one-time payments, or a continual source of revenue?