Where the primary focus of growth is on Detroit. Where the routeplanners sit down and say where are we going from Detroit next. There you go Detroit is less than 6% of the NK network and that is in no way committed to the Detroit market first and foremost. what portion of the SY network is associated with MSP. SY just announced around 10 new routes from MSP I can't ever remember NK announcing more than 2 or 3. But again we are talking history here and I understand SY is going in a different direction now so things are changing. NK announced a huge expansion at MCO just recently Detroit has never seen that level of commitment by NK. If DTW-MCI works why can't DTW-BNA or BDL.
I admire your dedication and fanboyism for your home market. I think it's great that you are so gung-ho about it, but for your benefit and if you want to be taken seriously, it's probably better to really take a moment to think about what you are writing, proof-read your posts and maybe even sleep on it so as to not be too emotionally charged when responding. It's much more fruitful to have a well-argued discussion than one that is oscillating between extremes and nears on excessive posting. You don't want to be the poster that is responding to every other post.
Under your logic, if airline X were based in DTW and flew one sole route, then they would be showing the level of commitment that you say is dedicated, since 100% of their resources are dedicated to Detroit; whereas Spirit only deploys 6% of their resources in the DTW market, yet has some 140 aircraft. Despite this you would say the level of commitment by airline X is greater than Spirit. I disagree with you, and furthermore, having all your eggs in one basket is very dangerous and can cause a company (or city for that matter) to fail. (aka Automobile Industry and Metro Detroit for a great example). Economic swings are extreme in Detroit if the Automobile Industry is struggling, so is Detroit. Thus diversifying your portfolio is good and this is what NK is exactly doing.
Spirit has been committed to the Detroit market for over 20 years, and was pushed out by NW who at Spirit's infancy was much smaller and financially weaker. Take a look at the DOJ court cases on predatory pricing between NW and Spirit on the DTW-BOS and DTW-PHL routes (I believe in 1996). Spirit then moved into another market (FLL) where they could build their business model and thrive. Now that Spirit is financially stronger and bigger, it has the wherewithal to battle Delta (ie new NW) in many markets. Thus you see an adaptation of Delta's LCC strategy in DTW. Just look at how DL reacted when NK entered MSP. DL cannot use so many resources to protect DTW, they are adapting to co-exist as NK, B6, AK, F9 aren't going anywhere.
NK is committed to the DTW market and has shown such commitment by investing capital to build a new hangar. As mentioned by other posters, their YOY growth has been impressive and is forcing Delta to be much more reactionary and upgauge markets. The competition and commitment that Spirit is showing in DTW is important because it shows that the market can be stimulated. This stimulation should finally become apparent in the passenger statistics in DTW. If Spirit can continue to stimulate traffic, I expect that DTW will have above industry (US) growth for an airport of its size since the last 20 years it's been relatively stagnant. If NK and DL continuously gauge up aircraft and routes, then you will see DL and other airlines add more routes. I believe there is a multiplier effect that occurs when this happens.
Thus I would be very thankful to Spirit for not only stimulating new customer segments that otherwise might drive or not travel out of state to stimulate traffic. They are showing the most commitment to your home market, and despite maybe not being the type of airline that you prefer due to their ULCC model, they are showing the most commitment, and their product is being best received.
Be patient and I think a lot of your 'wishes' for increased service in DTW will come true. I also believe that the increases will be sustainable and not hinged on incentives from local governments. I agree with you that DTW suffers a bit from underservice when it comes to other US carriers and international ones, but that's partially due to DL's fortress hub. As NK weakens the fortress, more service will come, but yields will fall as well and at some point the market will be saturated.