I 'can't stand' standing when I play the keyboard. Even in my early 20's it put considerable strain on my back because I've always required at least one foot pedal (sustain). To maintain healthy posture without developing back strain requires (at least for me) keeping both feet squarely on the floor when standing. Because I use sustain frequently throughout a song and throughout an evening gig, I position my keyboard height somewhere in between standing and sitting position, allowing me to sit on and/or lean against the edge of a high stool, of which helps support my balance and maintain body alignment when playing the pedal(s). Without the stool, it's like having to support my body all night on one (left) foot.

I hate singing in a seated position (even before my gut got rather large), so standing is a natural fix for that. Also, today's venues often have no stage, and I am entertaining on the same level as the audience - standing gives me a visual edge....also more freedom of movement. I have 2 footswitches at my right foot .. one for sustain, and the other for vocal harmonies. Sustain is pushed at least 90% of the night, so a quick shift to the right,and I turn on the "singers".
I put on and take off my guitar a few times each hour, so standing is very natural for me in that regard as well. The biggest reason is visibility though .... the audience can connect with me better and I have more control over the room dynamics when I can look out over the top ... or at least over their heads! lol
Back in the Rhodes days, I sat because I had no way to raise that beast up high enough an d still feel safe, but as digital replaces plywood .... everything just gets easier and easier.
Sit, stand - do what makes you happy, but make the music.

It's tougher on the body but I stand. Using wireless Headset mic and wireless acoustic guitar switching back and forth depending upon the song. I also find that standing allows me to sing better and on tunes where I am playing guitar I can get up close and mingle with the audience and dance/sing with them.
Losing about 40 pounds has made standing easier too! Whatever works for you.
Brian