is quickbooks likely to amend its software to allow CT600 forms to be submitted to HMRC after it closes its online portal. if not then why I should continue using it?

The HMRC's announcement will take effect next year, info2138. The management is already negotiating with them to ensure that QuickBooks can still provide the electronic filing of forms on the effective date.

While we are not yet on the list of the commercial software suppliers, I'd recommend checking out this link to review the conditions that fit you business setup: File your company's annual accounts with Companies House.
 

To continue using QuickBooks Online for the electronic filing services, we'll have to determine first three things:

• the type of accounts you're filing. Your accountant can help you with this.
• prepare the company authentication code.
• apply for a presenter account with Company houses.

The details are given in this article: Find software for filing company accounts. 

Once those three details are prepared, you can already file your forms with the Company houses.
 

If you need assistance with the Companies Houses, you can contact their support using this link: Contact Companies house. 

• Closure of the service to file your company accounts and tax return
  File your Corporation Tax return using Workpapers in QuickBooks Online Accountant
   

Let us know how the application for companies house goes. We'll be glad to give you pointers of what to do next based on what you want to happen.