I wouldn't call them understudies. An understudy is someone who practices and is prepared as a "back-up" in case the original proformer can not do their part for some reason. In the case of Steve he replaced Jim after he died, Eric replaced Frank when he steped back to focus on other things, and others were hired as replacements. Carol Spinney and Jerry Nelsons replacements were fased in slowly, and in a sort of an apprenticing way, but not all of them. These are not understudies, these are replacements, or maybe even second character actors.
I think all the replacements are good. It takes some time to get use to the new proformer but once you do you find they are just as great (if not better) than the original.
Below is a list of some of the major changes in character on Sesame Street
Ernie
Jim Henson 1969-1990
Steve Whitmire 1993-present
Bert
Frank Oz 1969-2000
Eric Jacobson 2000-present
Big Bird
Caroll Spinney 1969-present
Matt Vogel 1999-present
Grover
Frank Oz 1969-present
Eric Jacobson 1999-present
Cookie Monster
Frank Oz 1969-present
David Rudman 2000-present
Elmo
Brian Muehl 1979-1981
Richard Hunt 1981-1985
Kevin Clash 1985-present
Mr. Snuffle-upagus
Jerry Nelson 1971-1979
Marty Robinson 1980-present
Telly Monster
Brian Muehl 1979-1984
Marty Robinson 1984-present
Barkley the Dog
Brian Muehl 1970-1984, 1991
Fred Garbo Garver - 1984-1988
Bruce Connelly 1988-present
Betty Lou
Lisa Buckly 1970-1985
Fran Brill 1985-Present
Two-Headed Monster
Jerry Nelson and Richard Hunt 1980-1992
David Rudman and Joey Mazzarino 1999-present
Kermit the Frog
Jim Henson 1969-1990
Steve Whitmire 1990-present
Gladys the Cow
Richard Hunt 1970-1992
Jennifer Bernhart 2003-present
Grungetta
Brian Muehl 1982-1984
Pam Arciero 1984-1999
Gordon
Marty Robinson 1969-1971
Hall Miller 1971-1973
Roscoe Orman 1973-present
Miles
Miles Orman 1987-1992
Imani Patterson 1992-2002
Olamide Faison 2003-present