The reason it's a two-note slur instead of a three-note slur is:Let me know what you think – which elements of written music are open to interpretation? And if you have your own debate about something I wrote, I'd love to hear about it and make a ruling.
1. That same rhythmic figure appears a two-note slur throughout the entire piece (practically every odd-numbered measure) so playing it that way at 59 keeps things consistent. It's an essential part of the piece's DNA. Changing it on the last rhythm of the final measure would be weird.
2. Playing a two-note slur at m. 59 ensures that the last note is played with a down bow, which naturally has more power than an up bow and results in a natural accent.
3. Playing a two-note slur and ending the piece on a down bow keeps things consistent with the violins and violas, who also end with a down bow. Everyone ends together on a strong down bow, which gives the piece both a sonic and visual sense of finality.
4. I'm the composer and I know what I'm doing.
There's plenty of room for interpretation when it comes to expression, style, and even flexibility in tempo and dynamics (within reason) but notes, rhythms, and bowing are all carefully chosen and shouldn't be messed with. Especially in this piece, which requires precise, articulate playing.