It's pretty easy to run an alter table ... add index ...
If a team running a database can't or doesn't do that, when it's relevant, they don't know how to operate a database, or how to learn how to operate things. And they probably have no friends who know either.
That doesn't sound like a team where 'be sure to have an index on your DB' is going to be helpful.
Otoh, maybe they never bothered to add an index because they had no users, and unindexed queries were good enough while they developed their product that nobody wanted anyway. Table scans on 100,000 row tables aren't that slow when the whole table is in memory.
Adding indexes late isn't a huge deal. You add the index and things get faster and you go on with life. Worst case, you have to rebuild your table(s) so that indexes can apply... That could be a little rough, but probably not that rough until you have a lot of users; and if you have a lot of users, great!
If a team running a database can't or doesn't do that, when it's relevant, they don't know how to operate a database, or how to learn how to operate things. And they probably have no friends who know either.
That doesn't sound like a team where 'be sure to have an index on your DB' is going to be helpful.
Otoh, maybe they never bothered to add an index because they had no users, and unindexed queries were good enough while they developed their product that nobody wanted anyway. Table scans on 100,000 row tables aren't that slow when the whole table is in memory.
Adding indexes late isn't a huge deal. You add the index and things get faster and you go on with life. Worst case, you have to rebuild your table(s) so that indexes can apply... That could be a little rough, but probably not that rough until you have a lot of users; and if you have a lot of users, great!