The parts may be available, but Surface Mount components require much more specialist rework stations rather than just a soldering iron and a solder-sucker. The VLSI chips often used also don't lend themselves to signal-tracing for fault-finding, as it's near-impossible to know whether a chip itself is faulty, or one of the support chips it needs to work. Then there's the issue of adequate service documentation, so no, new amplifiers, of whatever class aren't user-repairable and may not even be manufacturer-repairable other than by board replacement, costing as much or more than just buying a new one. Once the warranty has expired, it's pretty much a throw-away item, and even in-warranty, if the user has to ship the item back to wherever, shipping costs may just not make it worth bothering with.
The business model now is to make it cheap and throw it away. Fortunately, that's also been accompanied by a large increase in reliability, but it doesn't help if one's unlucky.
S.