Sixt will probably have to cheapen their product or leave the United States. Unlike the other major firms, they do not have a discount/business-oriented brand such as Budget, Dollar Thrifty, or Enterprise to support them along the lean periods.

It's quite obvious that Sixt is way too cheap for the product they're offering. Hertz and Avis have average revenues in North America of $46/day (so about a $60+ price for the consumer) and probably give out on average a $30,000 car for that price. Meanwhile, you can book a $40,000+ LCAR at Sixt for about that same price most of the time. On top of that, Sixt gives one or two class upgrades and doesn't restrict the upgrade booking class to FCAR like Hertz. Sixt undercutting the big 3 is obviously unsustainable.

Hertz and Avis have both had dismal stock performance recently. Sixt USA has probably had it worse. To be profitable and sustainable, rental car companies will one day realize that they have to reduce the upgrades they hand out in order to survive. The most profitable customers should always be willing to pay for what they want (like Zipcar or Enterprise). There's no way a company like National is not losing money by giving me an upgrade to an SFAR for a US$20/day rental if I decide to put even 100km on it.