I never quite got the concept of original instruments. After all, the main thing that changed about modern instruments is they play better in tune. A modern flute plays much better in tune than old ones did. Brasses with valves play in much better tune than old ones without them. So why am I supposed to like people playing out of tune?
There is actually a bit more to it than that. They play with a gentler, more intimate sound. Violins had shorter bridges which created a less penetrating tone. Flutes were wooden and mellower. The players had to work harder to keep the intonation working, and this was easier to do without the blasting sound of a modern orchestra. If it's easier to keep in tune, it's easier to amass larger and larger orchestras, as happened throughout the nineteenth century. Wagner is not possible without modern orchestral instruments.
Orchestras with original instruments are smaller.
The original instrument movement is creating a different approach to performance of music from the Baroque. I grew up on Handel by the Huddersfield Choral Society which was scaled about the same as the Morman Tabernacle Choir. Big choruses, big orchestras, loud, unornamented singing was what we heard then.
Thinking about Harnancourt's Matthew Passion and Cecilia's Proibita album tells me things are changing. The lighter weight opens the door for lighter, more authentic, more florid singing. The modern vogue for the Baroque is in many ways a journey into the past, an attempt to find out what all the excitement was about. There is wonderful passion in the music of the Baroque that we are blessed to begin to hear anew.
Harnancourt and Bartoli are leaders in this movement, but they are certainly not alone. Marc Minkowski is another name that comes to mind. It's a way of hearing music that in my youth we didn't know.