CD/SACD transport mechanisms are consumables like the stylus on a record player. That is true for ALL silver disc players not just those using UMK5 mechanisms. Eventually the Optical Pickup Unit will fail because the laser naturally loses power over its life and can no longer read the disc. Alternatively moving parts wear out ( typically the mechanism which switches the different wavelength lasers in an SACD/CD machine). Sadly most of worldwide production of genuine CD replay mechanisms and of CD/SACD ones has been terminated by the OEM manufacturers in the Far East. In effect optical disc replay is now a niche activity, not only in the world of music, but also in regard to movies and computing itself. So the availability of spares is shrinking and in many cases they are unobtainable items. Fortunately dCS has made a huge investment in keeping stocks to support repairs as far as is practical.
This is a fact of life which has to be accepted and is why many people ( I think the majority on this forum) have switched from playing silver discs to ripping to computer storage. Doing this and replaying them using a computer application such as JRiver MC would be your cheapest option as you can still use the digital processing of the P8i for PCM files >24/48 KS/s. You may need to use a USB to S/Pdif converter as the P8i has no USB input.
The latter point also emphasises that in the world of digital replay this is now a player from a previous era and that inevitably involves compromises in regard to current expectations.
That last repair was done by a local technician using parts cannibalised from a donor transport. As the parts would have already been worn their subsequent lifespan would have been unpredictable.
Other than adopting computer replay the only alternative would seem to be to replace the UMK5 transport but you will need to assess if the cost is economically viable in the circumstances.