The laser will most likely last a long time. A laser life is normally between 25000 and 50000 hours, that’s what I have read.
Also, the laser can be adjusted if there is a problem.
With the service manual as an "instructor" there are no worries.
Spare parts for DIY.
https://encompass.com/item/11039994/MARANTZ/991312100050S/
https://www.partstown.com/marantz/mrtz991312100050s
https://www.brelect.fr/index.php?page=produits.inc.php&lib=MECANIQUE+TRAVERSE+UNIT+
DENON%2FMARANTZ&code=991312100050S
or call your local Marantz service center for parts availability and price.
The electrolytic capacitors will not last forever, either.
I’m looking forward to putting even higher quality Elna in my Ruby when the sound starts to degrade.