Not all Thinkpads have magnesium frames, and it's not a new addition.
Build quality on Thinkpads depends greatly on the model, and generation. Some of the first Lenovo branded models were terrible, but the IBM models were not infallible either. I can't tell you how many times I've seen "CPU fan failure", which is grossly misunderstood.
Best advice, for consumer level laptops, if you spend under about $800, Asus will give you the most for the money, just make sure it's an Intel I series.
Consumer laptops are engineered to last through the warranty under mild abuse. They rarely have more metal than an aluminum can, and use lots of flashy plastic to make them look better than they are.
If you want a good laptops, you really want to look at small business/corporate grade laptops as they offer better return on your investment. These are the HP Pro line, Dell XPS, Thinkpad x and T series (NOT Ideapad), etc... Not always spectacular to look at, but are more durable, easier to work on, and they are known to get the job done. These are where you can get cheap parts and good customer service. Beware, these fall like rocks on the second hand market, which is where I prefer to buy them and where they make a ton of sense. Buy a 2 or 3 year old Thinkpad with a low end I3, add an i5 or I7, throw in your SSD and ram and you might have $300 invested. If it breaks, you can replace almost anything on it for $20-$40. You can often get 7 useful years out of one of these if you aren't gaming and can often upgrade them a few times.
The best are executive class laptops, these are the Lenovo Carbon, old Sony Vaio Carbon fiber, Mac, etc.. They are pretty durable and have good looks. These often top $2000, and while they are the best of the bunch, when something goes wrong, they are expensive to fix, and I've found the manufacturers do not bend over to help like you would expect (Apple is the exception, not the norm). Resell on these laptops remains high, but so do parts to keep them running. 5 year old Sony high end laptops can easily retain 50% of their resale value, Macs are even higher.
Keep in mind, on many executive class laptops, they are often single or dual year models, so parts from one generation will not interchange with newer ones. Like on Thinkpads, the batteries can swap between several years, not so on a Sony or Mac. They have a high turnover, less testing, and as a result, are not always as refined as people think. The hinges on the old Macbook Air is a good example (they ripped out). You don't see these problems on Thinkpads because they test like crazy and re-use the same designs over and over again.
I like executive class, I've had several, but for work, I MUCH prefer something I can afford to repair. Last time my Sony was damaged, it was 5 years old and Sony wanted $500 just for the part, labor was another $200. As a whole, the laptop was worth more, but every time it was damaged it was $50 or even hundreds of dollars. I did fix the Sony, I found a replacement used part for $150, but that's still more than I paid for the entire Lenovo.
