Does your word hold weight?
In the tech world, declining offers after accepting is not new. It’s a norm we have come to accept grudgingly. We prepare for it by keeping the second-best and third-best candidates ‘warm’. The best here is relative and in terms of job fitment. If you construe the best as a rank, then I am afraid that’s not how it works.
In many ways, it's an employee’s market, especially when their skills are in demand. Companies that rescind offers at the last minute are also called out, but nobody calls them unprofessional. Maybe a few people do. However, prospective employees who decline the offer at the last minute are always called unprofessional.
You can change your mind. You are entitled to it. However, how you handle the situation speaks volumes about who you are. How you do small things is how you do big things. Do you have the courtesy to pick up the phone, call the hiring manager, and explain your thought process? You may not need to do it, but do you care enough to try it?
The absolute worst thing to do is send a nonchalant message, close the communication channel, and write empty goodbyes.
Such behaviour forces the question of whether the entire interview experience was an act to get the offer only to unveil the real personality. Most of us do put up an act, but if the performer is nowhere close to the real you, then you are doing a massive disservice to yourself.
Ultimately, the most pertinent question is, “Does your word hold weight?” If it does not, few will trust you. This question should haunt you, and if the answer is no more often than yes, then I am afraid you are ngmi.