You will notice the title is a statement, not a question: Big Bang Theory is a victim of its own success, and without due care on the part of the script-writing team, the traditional mid- to late-season slump will run for the length of the series.
The original cast included Sheldon, Leonard, Raj, and the little one without the PhD, and the hot girl across the hall, Penny. There were recurring guest characters, but they were kept carefully as guest characters and nothing more (I assume carefully kept; it could have been a coincidence) leaving the focus on the core five.
Recently, the writers have included the above five, Amy, Bernadette, and Priya, rounding out episode seventeen of series four (The Toast Deviation) with the addition of Zack (an ex- of Penny’s, he of the wipe-clean menu fame), Stu (owner of the comic book store) and Kripke (a rock-climbing hominid).
A story, or two more simple stories involving the core five, plus one, can be pursued in the twenty-minute slot, but to nearly double that is foolish, unless being handled by a very tight mind with a purpose greater than providing a break between commercials.
— -spoilers — -
What did the audience learn this week? Penny still has a soft spot for Leonard, and Bernadette wishes she’d been more promiscuous. How has the story been advanced? It hasn’t, aside from building up to a dull Penny-Leonard end of season cliff hanger, and all that at the expense of nothing more than a chuckle. Perhaps the writers are stuck wanting to write a soap, but confined to situational comedy.
That spoiler tag seems pointless now.