All of them. Does Salesforce let you modify their software? Can you use the Saleforce software independently of salesforce? Did they look at your workflow and talk to your staff before writing their software?
Not even remotely comparable in terms of cost. Moving from a £20 a month VPS to a £100 a month dedicated server because you now have to have capacity for a thousand users and a million customers instead of the 100 users and 100k customers you originally planned for is nothing like as expensive as having to pay a fee for every single one of those thousand users.
Very few businesses are big enough to have scaling problems with competently written software and they are far too big to be potential customers.
If I google salesforce customisation, I get 120,000 results and bunch of ads from customisation companies. Your answer is technically correct, I can't edit the source code, but it's also wrong, because as a business I can customise Salesforce to my hearts content.
If you're selling to businesses, you need to give the business answer.
Your solution is open source, there are countless open source products that have died as people have stopped updating them. PhantomJS for one - now any project that uses that needs rewriting.
I've no doubt your solution is technically better, but that's not what people want, people want safety and reassurance, and you need to offer that.
I think you need to take a step or two back from what do you do to what do you want?
Are you looking for global domination, UK domination, £1m company to sell, £1m earnings, a better job, more security, something to do, a hobby.
They're all valid answers, but they affect the what do you do question.