DL could carve out a nice focus city operation and become the second largest carrier at AUS, because most routes they'd only be legitimately competing with WN. And as I often say, if given the choice between WN and DL, most business with frequent travel would probably choose DL because of its extensive global network compared to WN's niche domestic market (niche because its great for flying in the lower 48 and to the Caribbean/select Latin America destinations, but anything outside of that and you are SOL). Don't forget DL would also be bringing TATL service (likely to AMS) with this operation, giving business travels convenient overseas travel options. WN can't provide that because they refuse to codeshare with foreign airlines. Oh, and while we are on the subject of international flying, a DL focus city would potentially help with landing much coveted TPAC service down the road.
This all being said, DL will have to move pretty quickly here to establish a focus city, because you are right, WN isn't going to stand idly by and let DL take over AUS, so they will try to grow more. If that happens, they could get to the point where they become too dominant and DL would lose out.
I don't know what ultimately will happen at AUS, but again, I would be surprised if DL were putting in that nice of a SkyClub just for a mere spoke, especially when they are in 4th place currently behind all 3 of their biggest rivals.
Let's not overanalyze the SkyClub piece, down the road in MSY DL is building a brand new 2 level skyclub as well... and no one has talked about DL building a focus city there. Despite the fact that DL has a larger market share at MSY, and has recently added new seasonal flights to SEA and BOS. I think once DL actually opens p2p flights from AUS, a focus city will be a real possibility, but until then I think AUS will remain a large spoke.
I think you are underestimating the presence that WN, AA, and UA have in AUS, and the FF base each has in AUS. Let's not forget AA/BA have a flight to LHR, and I'm sure AA and UA have a ton of corporate contracts with companies based in Austin. Plus, you also can't ignore G4 and F9 here because they will drive down Y class yields on flights.
I think they are putting a new SkyClub at MSY only because the whole terminal is being replaced, as as such, they have an opportunity to design a new, larger one. Otherwise, they wouldn't be bothering with it. In the case of AUS, there really isn't a reason for DL to put a SkyClub period since they are in fourth behind WN, AA and UA (and don't have a big presence in Texas). Yet, they are anyway, and a pretty elaborate one too. It will be the only non hub with a Skydeck. This, combined with rumors about AUS-AMS potentially becoming a year round flight fuels the rumors of a focus city. We even had someone come on here and claim that DL will be adding quote "a lot!" from AUS soon. Of course, we absolutely need take that with a microscopic grain of salt, but nonetheless, that bold proclamation combined with the other things makes for some interesting speculation.
Indeed, WN, AA and UA all have strong FF base in AUS, since they all have large hubs/stations nearby. That probably explains why the BA flight has been so wildly successful. However, as I pointed out, none have an absolutely overwhelmingly dominant presence in AUS (there is about an 850,000 pax difference between AA and #2 and DL at #4, big, but not nearly as significant as some other airports). Also, in the case of AA and UA, one has to connect via DFW and IAH to get anywhere outside of their hub cities, it is highly unlikely either will offer p2p service in the future. If given the choice between direct flights on DL or connecting via AA and UA, I'm pretty sure DL will win out. Also, G4 and F9 don't have daily flights and their schedules are not consistent (routes come and go frequently). They are primarily targeted towards leisure travelers and a fair amount of their routes (particularly on F9) are seasonal. As such, I truly find it hard to believe they are a serious threat. No doubt, they will impact the yields on Y seats, but given the reasons I just stated, will it be overwhelming? I'm not so convinced.
So all in all, WN is the only significant obstacle to a DL focus city in AUS, and honestly, I don't have a good explanation of how DL would overcome that. That being said, I don't think they necessarily need to knock off WN as the leading carrier (unless in the unlikely event they decide to go full hub mode in AUS), but simply follow the strategy of RDU and CVG and provide routes to the most popular non-hub/focus city destinations plus add a TATL flight to AMS.
Again, who knows what will happen. Certainly, DL does not need a presence in Texas to thrive, they are doing more than just fine. However, of the US3's biggest holes in their respective networks (UA in the Southeast, AA in the Mountain states/Northwest and DL in Texas), I'd argue anyday that DL's is the most significant, as Texas is the second largest state in the union and the second largest state economy. AUS may be their last chance to gain some sort of a foothold in Texas. And if you think about it, it is the perfect place in Texas for them to set up an operation; a vibrant city that is among the fastest growing markets in the country with young professionals (DL touts itself as becoming the airline of choice among young professionals).
I'd really love to hear DL's thoughts on their lack of a presence in Texas (similar to how United addressed their lack of a SE hub), if anyone has some link to where they discuss that, please share it here.