The USA has had a few lean years of late in this event, so maybe Spearmon can inject some life into this event from a US perspective. However, I do agree that this is generally a young mans event (Michael Johnson aside) and Wallace may find he is a wee bit too late in moving up.
On the subject of him being a great finisher in the 200m, don't be fooled into thinking there is a direct correlation between 200m finishing speed and the 400m. See Marita Koch, often closed at the end of a 200m by other 100/200 sprinters, such as Ottey and Ashford, despite being amazing at 400m. The events are run at different maximum speeds over different distances.
I remember seeing Wallace run in the 4x400 indoors in the UK and before his leg I was thinking "this is it, he's going to blast a wicked leg" and in fact he was mediocre. If he doesn't overcome his mental block as others have pointed out, he will have trouble in this event. But as the great Jarmila Kratochvilova said, "it hurts whether you run a 49 or a 51
" so my advice to Wallace would be; dont be a pussy, just go for it