From 2000 to 70,000 users in 4 months - good or bad?

it depends on following

1. How much load are free users creating on your service and is it sustainable?
2. What is your conversion ratio?
3. How much are you spending in sales and marketing.

If some percentage of users are converting and it is profitable enough to keep free users than keep them else you will have to do some tweaking in your freemium plan to ensure that users convert.

I think the determining factor is how many you are converting to paying customers. If you grow to 70K users but no one pays, well... I would say something is not working in the funnel. Your instincts are correct: you want paying customers, especially from an investor standpoint. But even w/o investors in mind, the objective of any business is to turn a profit - so that should be where you spend your time.

All that said, congrats on the growth! Clearly there is an audience wanting what you're offering. Just need to finetune the onboarding/upselling a bit and you're in a good place!

Another thing to consider is are they using this service?
You have users but are they all active? Are they all engaged? What's your dropoff?

At the end of the day counting users only shows one story.
Look at the recent Twitter bot conversation in regards to users and "real user" count.

What is your plan to convert? As long as you figure one out and see the willingness from those free users, then fine! But you should start working on validating your hypotheses.

Have you considered asking your existing users. Is there something they would pay for right now? and/or Some kind of survey

How did you grow that fast?

Are these users bots or real people. If so, what traffic sources are they coming from?

What is your conversion rate from free to paid?

Was researching the freemium model recently because I am considering it for my own developer focused product to acquire initial users. But my impression is that it doesn’t really pay off, way too few converts. Perhaps even detrimental because the product isn’t being monetarily validated, essentially you might be serving a free lunch to users don’t actually want to pay (not the right target group).

Besides big tech companies, free tier is fairly rare.

There is no right or wrong answer to this question, as it depends on your specific goals and business model. If your goal is to grow your user base as quickly as possible, then freemium users can be a great way to do this. However, if your goal is to generate revenue, then you need to focus on converting freemium users to paid users.