I've been in this situation a couple of times before.
It won't make a blind bit of difference initially but if you expand, so will tooling and deployment costs and investors don't like that. Anything that takes off the bottom line is a problem. Microsoft licensing, particularly dev tools and SQL is incredibly expensive and an order of magnitude more expensive on deployment and diagnostic time than anything else.
We're like that now. Our SQL licensing is shaping our architecture not on technical merit but avoiding core license costs. That is beyond bad but we can't justify a £200k spend on licenses and kit to get rid of 2008R2.
Inevitably bits of Python are now appearing around the edges of the product and trying to stab the core. There are a couple of postgresql machines doing ancillary work. All the staff are slowly moving their operational task over because there isn't a purchase process.
This is the price of success though :)
Edit: just to add that the last two companies I worked for are Python/java/postgres houses now after being end to end MS outfits. The staff change over the lifecycle of a product. You can afford better people after time and better people seem to prefer to use other platforms even if they market themselves as .Net people.
Edit 2: also once you're locked in, volume licensing looks cost efficient. Then after a couple of yeara you get a call from the sales guys at MS who want to do a 'soft audit' (which it says they can so in the VL agreement). Then battle commences between MS, yourselves and the disty who handles your VL over how much you owe. This usually starts at £150k-ish for an SME and your ops team will be down for days whilst they work that down. Our liability was less than £10k which is within the VL safe zone but other companies are less lucky. Also the VL settlement says you can't discuss it but fuck them - the company this was against is dissolved.
Actually I look at my post and I couldn't possibly recommend it. If we'd started differently we could afford to be employ and sponsor postgres core team staff for example and gave second to none support and a positive impact on the community. Instead, pockets are lined elsewhere.