DVD/HDD recorders are basically cobbled-together from proprietary pieces that don't mesh well, in an effort to make a "simple" standalone consumer product. That they work for us at all is a minor miracle, because recordable DVD at its heart is a "computer+hard drive+elaborate software" medium. Blank DVD formulations have been a moving target since day one, requiring easily-upgraded or replaced burners and software to keep up. Standalone recorders are a paradox, in that they are completely hostile to these requirements. They use burners that can't be easily replaced, and burning routines that can't be updated, because of Hollywood piracy concerns. Sooner or later, usually sooner, parts of the recorder become obsolete or worn, and problems start.
Recorder burners don't generally fail catastrophically: they lose one function after another, and not always in ways you'd expect. High speed dubbing from HDD to DVD puts far more momentary stress on the laser assembly and mechanics than real-time recording, so this is the function that tends to fail first. The same disc in the same machine may indeed work fine for direct 1:1 recording, because it is slower and puts less peak stress on the system moment-to-moment. But this too eventually fails, because real-time operation uses up the remaining laser lifespan faster than quick high-speed dubs from HDD. Also the majority of DVD blanks made today are not designed for real-time recording, it works as sort of an afterthought but the discs are really optimized for high-speed HDD>DVD burning- even the TY. Then you have the brand and model specific recorder idiosyncracies: Sonys made before 2007 are famous for early burner failure with demented "dirty disc" alerts. Its best to look on the bright side, and be glad you got an extended use from your own Sony.
Unfortunately the standalone DVD recorder has always been a sort of half-assed concept. No thought was given to consistency of media mfr, or how inane anti-piracy measures would obsolete a proprietary recorder the day after it shipped. For a variety of reasons, including expensive hard to use initial models, DVD recorders never really caught on in North America in sufficient numbers to stabilize the demand. Within a couple years, consumers largely ignored them, and DVD burning became something done on a PC (primarily, ripping of movie or game discs). PC users wanted cheap fast drives and media, thats where the mfrs went, and standalone recorders became orphaned products prone to wearing out or unable to burn current media. So when a recorder like your 715 starts failing, there are no repairs or exact replacements available. Its just how things played out in the marketplace: those of use who prefer standalones are a minority with few options.