Literally nobody is going to care about occasional bus upsets that might or might not happen in a toy robot.
And with properly dimensioned pull-ups or a current source, standard-rate I2C runs just fine over a few meters in a regular home/lab environment.
You think? When such "bus upset" freezes the robot/gadget because the hw I2C peripheral is now stuck waiting for that clock line to be released? Or worse, when it causes the motor to keep spinning until something breaks because the micro is unable to read some sensors?
Cheap DC motors like those in those servos are
terrible when it comes to EMI. I had one of those cheap yellow plastic gear motors in a project and without a boatload of extra noise suppression components (two chokes, several capacitors, ferrite ring ...) there was absolutely NO chance to run an I2C line to a sensor nearby. And we are talking about maybe 15 cm of wire, not a 50cm+ long antenna you may easily get in a small robot once you actually dress the wires properly.
That I2C runs "just fine over a few meters in a regular home/lab environment" means squat all about running in an environment where you have several brushed motors spewing EMI in one place, along with a wifi or some other radio and a bunch of sensors.
This is a terrible design decision even for toys and will give the user no end of grief. And the OP a lot of customer support/returns/bad reviews.
RS485 would cost literally peanuts to add and would work much better - all you need is a transciever (few cents) and an UART on both sides of the link. Oh and you can actually make a multidrop bus with RS485 too with some care (even though then CAN is probably better).
There is no RS-485 or CAN driver on any regular Arduino, NodeMCU or Blue Pill board.
That's wrong. Blue Pill certainly has CAN (but you can't use it with USB at the same time). Also the more recent Black Pill has CAN peripheral, I believe. You only need a transciever like MCP2551.
Arduino Due (ATSAM3X8E) has 2 CAN peripherals built-in, possibly others. There are also CAN shields available. You can also attach a dedicated CAN controller that talks to the host over SPI if the host doesn't support CAN natively - like MCP2515.
This is literally all you need to make CAN work even on a lowly Uno:
https://www.mikroe.com/can-spi-board Two chips + a few passives, maybe $5 in parts.
RS-485 does not need any special hardware support, only UART which all of the above have - you just need to add a transciever IC. Those cost peanuts and are typically a simple 8 pin device. Heck, there is even an official Arduino shield with an RS485 transciever.