Keep in mind that both actors, Kermit Love and Chet O'Brien, were on the production crew of Sesame Street. Love (Willy) was a puppet designer/builder and O'Brien (Macintosh) was the floor manager and they were given small roles on the show. I don't think they were ever meant to be very prominant characters the same way Bob, Susan and Gordon were. I think Macintosh did a little more, but they were just little bit roles. Kermit Love was old, but probably not much older than Will Lee (Hooper). I believe he is still alive today but in his 90s. Hope this helps at all.
As for Christmas Eve On Sesame Street, Mr. Macintosh had a cameo but you're right he didn't say anything. I am also realizing that David and Olivia weren't in much of the special either. Olivia was featured in the "True Blue Miracle" number which big but after than her and David had no lines and were not seen again until the finale. The only ones who had major roles in the special were Gordon, Susan, Mr. Hooper, Bob, Linda and Maria. Olivia and David sort of went the way the Count did. The Count was in the beginning with the ice skating stuff and seen during that opening song, but he is the only Muppet we never see again doing anything after.