After drinking alone and with shamans, I will say - shamans certainly add something very tangible and distinct to the experience and add a much deeper level of work to the ceremony for me.
They are in no way necessary at all. Very helpful in every way, but not necessary. Kinda like when learning guitar - you dont need a teacher at all, but you can learn to play better faster if you have a good teacher, and the better the teacher, the more you will learn. Nothing is wrong with teaching yourself, and nothing is wrong with learning from the deep wisdom of someone more experienced.
For some people a shaman might be a bit more necessary - for example if you are trying to cure epilepsy I think it would be better to sit with someone who has lots of experiences safely guiding people because of the danger involved. I know of someone who cured his epilepsy with Ayahuasca BTW - a friend of mine.
If someone has cancer and might die in the next year I think they are better off visiting a shaman then going it alone - just because the shaman can take them deeper faster. They have a lot to lose, so maybe can benefit from someone who has decades of experience (the shamans I stayed with in Peru usually had over 30 years of experience as a curandero, which is a lot of knowledge!).
And in the Amazon at least, it isnt just about drinking Ayahuasca.... Many locals dont even drink it when they go to ceremony! There is also the dietas, secondary plant medicines, energy work, massage and bodywork like bone-setting, vapor treatments, flower baths, icaros ect.... I have seen some people heal without ever taking a sip - they just had the shaman work on them.
But if you are just trying to learn about the spirit world, experience Ayahuasca, or in search of healing for less dangerous problems, you are probably fine drinking alone if that is what you feel called to.