It depends on the tires.
No Lexus comes with the Toyo Proxes TPT radials. The Toyos the ES comes with are Toyo Proxes summer tires (J33). Summer tires, whatever the brand, will always wear out faster than all season tires. Its unfair to compare a summer Toyo with an all season Michelin when it comes to treadwear. The Proxes TPT are an all season radial with higher treadwear ratings than the Michelin MXV4.
You can't go wrong with the MXV4s they've proven themselves a good tire time and time again. The question is are they worth the cost. Many shops ask over $200 a tire for the MXV4s mounted and balanced, thats just insane whereas the Proxes can be had for $100-$130 a tire. Toyo has been the quality award winner something like 5 times in a row, and the Proxes TPT has been cited as their flagship, highest quality tire. I'm 100% satsified with mine, the tires made all the difference in the car. They're great looking tires too. Michelin is #2 in the quality ranking and the MXV4 isn't even on the chart.
15,000 miles is not unusual wear for a set of summer tires. The TPTs have a 60,000 mile treadwear warranty. Fact of the matter is, stock tires just suck. Owners of cars from all manufacturers complain about crappy stock tires. Michelin makes cheap tires with no treadwear warranty also, doesn't mean their higher line warrantied tires are bad tires. I've never had a car that came with decent tires, I always run the stock tires for 15-20k miles and replace them with good tires.
The reason for your dissatisfaction is you're comparing your experience with the stock ES Toyo summer tires with all season Michelin tires. Thats like buying an SUV after owning a Corolla and complaining about the gas mileage difference. Apples and oranges.