Brandon, valid concerns. Integrated fixtures exist because they are can be very robust solutions - better than drop-ins for a variety of reasons. Modular integrated fixtures are even better - replaceable drivers, LED arrays, and optics - but, you're right - can you trust a manufacturer to support their modular components after many years? Check the manufacturers' reputation for standing by their products and their committment to the landscape lighting profession in the long term.
If you're committed to providing the best products to your best clients, do your research and select fixtures that are specifically engineered for LED light sources.