Well he's not alone with that, and letting go of companions was as much a chore for RTD as Moffat, and once you get into the realm of people wanting to travel with the Doctor forever you're stuck with how to handle that when the actor wants to leave or the character's run his/her course. So Rose had to be separated from the Doctor by being trapped in another universe she could never, EVER, escape (until she did) Donna had to undergo a mind wipe, Amy and Rory had to be trapped in the past, Clara had to die/be reborn/become immortal/never see the Doctor again, Bill became a Cyberman and then a crazy alien water thing...really the only two modern companions whose departures felt like classic departures were Nardole (very Nyssa) and (ironically given she's perhaps my least favourite character) Martha, and even Martha had to come back several times. Note I'm kinda ignoring Jack and River here, but realistically we did see both their exits. The Face of Boe and Professor River Song both died.
On one level I get it, the Doctor has full control of his Tardis and back in the day he was somewhat adept at abandoning people and never, ever seeing them again, but the modern show's gone too far the other way. Everyone always wants to travel with the Doctor and he can always get them home again and pop round for tea long after they've left.
There must be a middle ground.
I'd like to see more companions who travel with the Doctor almost by accident (Tegan, Ian and Barb) or because they have nowhere else to go (Nyssa, Turlough) and it'd be nice if just sometimes people had had enough. Tegan's exit is still phenomenal, and it's surprising more companions don't effectively get PTSD. Yes they see wonders, but a whole lot of warzones too!
I still liken travelling with the Doctor as being akin to backpacking on your gap year. It's fun and adventurous, and sure some people turn it into a life, but most people get tired of sleeping in tents, tired of the lack of stability, and they want to go home get a job raise a family etc.