No, that does not appear to be accurate. McCain got 45.7% of the vote, while Trump got 46.1% which is less than a 1% difference. Meanwhile, the total turnout in 2008 was 58.2% compared to 55.7% in 2016. So no, there was no depressed Republican etc., turnout in 2008 that you can blame McCain's loss on. If anything, it was increased Democratic turnout and maybe some defection to the Democrats that made the difference in 2008. Ultimately, the highest Republican turnout over the last 5 cycles would not have been anywhere near close to enough to beat Obama in 2008... and that belies the untapped power that the Democrats are sitting on, in terms of their apathetic electorate.
The reality is that the Republican electorate is pretty static and consistent, while the Democrats turnout is prone to huge swings, depending on how energized/inspired/motivated they are. Baby Bush got 62 million+ votes in 2004... fast forward to 2016... Trump got 62 million+, with McCain and Romney both hauling in just under and just over 60 million respectively in the years between. Meanwhile, the Democrats got about10 million more votes in 2008 than in 2004,, while getting about 4 million less votes in 2016 than in 2008. The Republicans have nothing remotely resembling that in potential votes gained or lost.
Which recent losing Democratic candidate would you describe as having given a "respectable showing" rather than as a "failed candidate"? Or is that distinction reserved for Republicans in your mind?