I'd struggle to see much of a case. The leisure demand for travel from London to Scarborough won't be huge - there are far closer seaside towns for Londoners and much of the North York Moors (where some of the leisure demand would be going) is just as easily reached via York or Middlesbrough. Scarborough is really more of a regional destination (which isn't to belittle it - lots of people definitely want to go there, but mainly from the North East or Yorkshire). Realistically the only LNER stations with significant flows to Scarborough would be York, Doncaster, and maybe Retford/Newark/Grantham (though from there Skegness is just as likely to be a draw). The former two both already have direct Scarborough services (albeit Doncaster's is slow), while the latter three aren't exactly huge.
On paper, EMR's service was better than any LNER service could ever be. Sheffield is bigger than Doncaster (in both cases competing with the slow Northern service); Leicester, Chesterfield, and Derby are bigger than Peterborough, Grantham, Newark, and Retford (for new direct links); and it had the potential to also pick up direct customers from Leicester or points south to York who otherwise have to change (whereas an LNER service along the East Coast doesn't introduce new links). I'm open to hearing a compelling argument as to why LNER would do better than EMR but until that point I just can't see this hypothetical service working well.