A trombone should last not just for a few years, but rather for a few decades. Nowadays, materials, manufacturing processes, and coatings ensure that instruments are highly durable.
Many manufacturers produce all parts of the instruments entirely by hand, which, of course, comes with a price. Others have parts made for them or purchase components like the quart valve and assemble them by hand. Industrial production of a complete instrument is also possible. However, handmade instruments are often said to have better craftsmanship and quality.
In addition to manufacturing quality, good response and handling are crucial for choosing an instrument. There are often good beginner instruments available at an attractive price.
But one thing to always keep in mind: Buy from a specialty store!