The original version of Ultima I was written in a mixture of BASIC and assembly. BASIC is pretty slow, but most BASIC implementations let you call into optimized assembly routines.

Though, past a certain point of complexity, performance aside, assembly might be more readable than BASIC because BASIC relied on line numbers for jumping around, whereas assemblers offered named labels.

I think there were a lot of small commercial applications if you count programs you could order from the little ads in the back of magazines. BASIC was fine for something like a recipe or address database, that didn't involve any graphics or sound.

BASIC had great facilities for both graphics and (matching the hardware of that era). Various simple primitives like LINE and CIRCLE baked into the language itself, the PLAY statement, ON PEN for light pens and ON STRIG for joysticks. There were quite a few video games for the likes of C64 and ZX Spectrum written in BASIC, usually with machine code snippets for perf-critical stuff.

Depends on the version of BASIC. The BASIC 2.0 in the C64 didn't have those things, but 7.0 in the C128 had graphic and sound commands. Unfortunately, the C128's near-100% compatibility with the C64 meant that very few games were written to take advantage of its new abilities. It made more sense to keep making games for the C64 and its much bigger market.

Enough small businesses wrote little BASIC programs that Visual Basic was a huge deal for small businesses at the time. There were a lot of business apps written in VB. There are still a weirdly high number of business apps written in VBA for Access and Excel each year in some industries.