for context i've been my best friend's favorite person for a very long time now. they seem to be growing more attached to me lately, giving me constant affection and showing me images that remind them of us. 

a few days ago i went offline for a day due to IRL stuff and i accidentally forgot to respond to their question asking me if they were making me uncomfortable with the constant affection. they told me they felt paranoid that i hated them and was ignoring them, to the point they left our Discord server and started venting on other sites. i came back the next day and told them truthfully, while I'm okay with some small doses affection constant affection bothers me since I'm a very emotionally closed off person. they accepted that and told me they understood.

now this morning they've asked me if we can be best friends and platonic soulmates forever and told me they never want to lose me. i have absolutely NO intention of leaving them, but the logical side of my mind is telling me that i can't promise that we'll ever lose contact. once i start college and actually start having a life i don't know how much time I'll have for completely online friendships. i want them to be able to live without me.

how should i respond to this?