My aunt has Down's and is quite mild. She used to work as a carer though has now retired (she's 70), she lives independently in a council flat where she manages her own spending, shopping, cooking etc. She doesn't drive but is fine using buses. She even goes on coach holidays by herself. She can read and write, though finds both quite hard. However, I know that it took a lot of work from her parents and sister to enable this level of self-reliance, and she still needs a lot of help. We have to set up and manage her accounts, her technology, her bank accounts etc, and always be on hand to offer any assistance as and when she needs it. Now her parents are gone, her sister helps enormously, and I'm doing more and more. I know I will be relied on increasingly as her older sister becomes less able to help, and expect to assume full responsibility for her in the next decade or so.
So even if your child is on the milder end of the spectrum, he/she will nevertheless need a huge amount of time, energy and money spent to allow her to live semi-independently, and this will need to continue after you are gone. If you have no other children, there won't be that automatic support network - and even if you did, there's no guarantee that they would be willing or able to offer their sibling lifelong support. My aunt has me, plus four nephews. None of her nephews offer any help.
Plus, I wouldn't count on the state fulfilling all their needs. My aunt was fortunate to be born when there was a real effort to help disabled children become fully-functioning members of society, so far as was possible. These days there seems to be significantly less interest in putting the work in, and so just put people like her on benefits instead. She had special schooling which meant her own needs were prioritised, whereas now she would have been stuck in mainstream education, falling hopelessly behind and feeling like a total failure.
She is very happy, and very much loved. There is obviously no question at all that given the choice, any of us - most especially her parents - would be without her. It will be a huge amount of work and a constant worry, but I don't believe you will ever regret it. However, though it feels evil to say, unless you have a great deal of money and a vast support network, I would terminate.