First of all there are joint projects with India, France, Italy, China, etc... on weapon systems that are in demand by both militaries, and/or in 3rd countries. In this case, exporting such weaponry is not only up to Russia, but also to its partner in development.
Many other weapons are geared specifically for export and are not required by the Russian military; yet without sales and demand for these models they wouldn't sell and none would be researched and built apart from that which the Russian military demands. Which would deprive Russian specialists and holdings from very useful development experience and results; that could perhaps come in handy later or could be used to further enhance systems that are required by the Russian MoD.
Many of the weapons systems that the Russian military does demand would be unjustifiably expensive if exports of them were prohibited; as the Russian MoD orders alone might not be in high enough digits to create a viable economy of scale.
Thus the result would be that Russia would have to import quite a few of its military hardware.... from countries that DO export weapons.
Either way, you can't win - arms industries will continue to profit it's just a matter of whether it's yours or the other guy's.