semiconductors, resistors, capacitors etc don't have MTBF (not in our lifetime). They operate practically forever (except electrolytic caps).
Not true at all, unless you are referring to a single part operating by itself, which you are not. The combined mtbf of the hundreds or thousands of semiconductor and passive devices (even excluding electrolytic capacitors) in a typical sophisticated audio system will be measured in decades at most, and quite conceivably in years (less than one decade). That is especially true when you consider the sharp increase in failure rate that occurs with older components, according to accepted reliability models, and is even more true when you consider that many amplifier designs intentionally run semiconductors at high junction temperatures in the interests of improving the sound quality.
Please take a quick look at some of the 205 pages of MIL-HDBK-217F, linked to below, which is the guideline document for reliability calculations for military systems. Note that it addresses just about every conceivable type of electronic component. And keep in mind also that these are typically components that are manufactured, burned in, tested, and screened vastly more rigorously than anything in most consumer audio systems.
http://assist.daps.dla.mil/docimages/A/0000/0005/3939/000000041349_000000020839_BTJRBUHXWM.PDF?CFID=18958726&CFTOKEN=fdbcdd5971174382-FEB22389-1372-548A-D368EF4CD631E514&jsessionid=063010dbe387801aa415
Regards,
-- Al